ARM-21N2 6DOF Robot Arm to move in X,Y,Z Coordinates

Question

I use ARM-21N2 6DOF Robot Arm from https://www.thanksbuyer.com/arm-21n2-6dof-robot-arm-kit-metal-robotic-arm-mechanical-arm-unassembled-without-servo-frame-only-75929. Any idea how to move it in a straight line for X, Y, and Z axis? The value that I input on textBox_XStartingpoint will be the initial position of the end effector, and the value that will be input on textBox_XEndingPoint will be the final position of the end effector but it should move in a straight line, same will goes to Y, and Z axis.

Thanks Ahead! Here's my code for arduino

#include <Servo.h>

//Global Variable
char c;
String dataIn;
uint8_t servo1Degree, servo2Degree, servo3Degree, servo4Degree, servo5Degree; 
int8_t indexOfA, indexOfB, indexOfC, indexOfD, indexOfE;
int speedDelay;

Servo baseServo;
Servo shoulderServo; 
Servo elbowServo;
Servo wristServo;
Servo wristRollServo;

const int basePin = 5;
const int shoulderPin = 6;
const int elbowPin = 7;
const int wristPin = 8;
const int wristRollPin = 9;

void setup() { 
  Serial.begin(57600);

  baseServo.attach(basePin, 500, 2500);
  shoulderServo.attach(shoulderPin, 500, 2500);
  elbowServo.attach(elbowPin, 500, 2500);
  wristServo.attach(wristPin, 500, 2500);
  wristRollServo.attach(wristRollPin, 500, 2500);

  servo1Degree = 90;
  baseServo.write(servo1Degree);
  servo2Degree = 30;
  shoulderServo.write(servo2Degree);
  servo3Degree = 50;
  elbowServo.write(servo3Degree);
  servo4Degree = 90;
  wristServo.write (servo4Degree);
  servo5Degree = 180;
  wristRollServo.write(servo5Degree);
  delay(2500);
}

void loop() 
{
  // put your main code here, to run repeatedly:
   Receive_Serial_Data();
  if(c=='\n')
  {
    Parse_the_Data();
    c=0;
    dataIn="";
  }

  baseServo.write(servo1Degree);
  shoulderServo.write(servo2Degree);
  elbowServo.write(servo3Degree);
  wristServo.write(servo4Degree);
  wristRollServo.write(servo5Degree);
}

void Receive_Serial_Data()
{

  while(Serial.available()>0)
    {
      c=Serial.read();
      if(c=='\n') {break;}
      else        {dataIn+=c;}
    }
}

void Parse_the_Data()
{
   String str_servo1Degree, str_servo2Degree, str_servo3Degree, str_servo4Degree, str_servo5Degree;

   indexOfA = dataIn.indexOf("A");
   indexOfB = dataIn.indexOf("B");
   indexOfC = dataIn.indexOf("C");
   indexOfD = dataIn.indexOf("D");  
   indexOfE = dataIn.indexOf("E");

   if(indexOfA > -1) { str_servo1Degree = dataIn.substring(0, indexOfA);            servo1Degree = str_servo1Degree.toInt(); }
   if(indexOfB > -1) { str_servo2Degree = dataIn.substring(indexOfA+1, indexOfB);   servo2Degree = str_servo2Degree.toInt(); }
   if(indexOfC > -1) { str_servo3Degree = dataIn.substring(indexOfB+1, indexOfC);   servo3Degree = str_servo3Degree.toInt(); }
   if(indexOfD > -1) { str_servo4Degree = dataIn.substring(indexOfC+1, indexOfD);   servo4Degree = str_servo4Degree.toInt(); }
   if(indexOfE > -1) { str_servo5Degree = dataIn.substring(indexOfD+1, indexOfE);   servo5Degree = str_servo5Degree.toInt(); }   
  
}

and here's the code to the UI

private void textBox_XStartingPoint_KeyDown(object sender, KeyEventArgs e)
        {
            if(e.KeyCode == Keys.Enter)
            {
                string theta1 = textBox_XStartingPoint.Text;
                if(int.TryParse(theta1, out int horizon))
                {
                    if(horizon >= 0 && horizon <= 180)
                    {
                        trackBar_BaseServo.Value = Convert.ToInt32(horizon);
                        textBox_XStartingPoint.Focus();
                        if(serialPort1.IsOpen)
                        {
                            serialPort1.Write(horizon + "A" + "\n");
                        }
                    }
                    else
                    {
                        MessageBox.Show("Value must be betwee 0 to 180");
                    }
                }
                else
                {
                    MessageBox.Show("Please input valid integer only");
                }
            }
           
        }

        private void textBox_XEndingPoint_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Enter)
            {
                if (int.TryParse(textBox_XStartingPoint.Text, out int startBaseAngle) &&
                    int.TryParse(textBox_XEndingPoint.Text, out int endBaseAngle))
                {
                    double L1 = 8.3; // Length from base to elbow
                    double L2 = 14.8; // Length from elbow to wrist

                    // Assuming Y is constant for a horizontal line (Replace Y with the actual height)
                    double Y = 0;

                    // Assuming X is a function of the base angle, calculate initial X
                    double initialX = 0; // Replace with a calculation or value based on startBaseAngle

                    for (int baseAngle = startBaseAngle; baseAngle >= endBaseAngle; baseAngle--)
                    {
                        // Update X based on baseAngle
                        double X = 0; // Replace with a calculation or value based on baseAngle

                        double theta1, theta2;
                        SolveInverseKinematics(X, Y, L1, L2, out theta1, out theta2);

                        if (serialPort1.IsOpen)
                        {
                            // Send angles to servos
                            serialPort1.Write(baseAngle + "A" + "\n");  // Send to Base
                            serialPort1.Write(((int)theta1) + "B" + "\n");  // Send to Shoulder
                            serialPort1.Write(((int)theta2) + "C" + "\n");  // Send to Elbow

                            // Add a delay for servo movement (optional)
                            System.Threading.Thread.Sleep(100);
                        }
                    }
                }
                else
                {
                    MessageBox.Show("Please input valid integer only");
                }
            }
        }

        private void textBox_YStartingPoint_KeyDown(object sender, KeyEventArgs e)
        {

        }

        private void textBox_YEndingPoint_KeyDown(object sender, KeyEventArgs e)
        {

        }

        private void textBox_ZStartingPoint_KeyDown(object sender, KeyEventArgs e)
        {

        }

        private void textBox_ZEndingPoint_KeyDown(object sender, KeyEventArgs e)
        {

        }