collect2.exe: error: ld returned 1 exit status (error in c)

Question

I wrote three programs

this is main:

#include <stdio.h>
#include "head.h"

int main() {
    printf("Hello, World!\n");
    int z = mult(3,5);
    printf("%d", z);
    return 0;

this is head.h:

#ifndef EX0_HEAD_H
#define EX0_HEAD_H
int mult(int x, int y);
#endif //EX0_HEAD_H

this is head.c:

#include <stdio.h>
#include "head.h"

int mult(int x, int y)
{
    int z = x*y;
    return z;

and the cmake file:

cmake_minimum_required(VERSION 3.24)
project(ex0 C)

set(CMAKE_C_STANDARD 99)

add_executable(ex0 main.c head.c head.h)

i need to know why i got this error:

gcc.exe -o main "C:\Users\jonat\OneDrive\שולחן העבודה\c&c++\ex0\main.c"
C:\Program Files\JetBrains\CLion 2022.3.3\bin\mingw\bin/ld.exe: C:\Users\jonat\AppData\Local\Temp\ccsmetoW.o:main.c:(.text+0x78): undefined reference to `mult'
collect2.exe: error: ld returned 1 exit status