Passing variables into command line arguments

Question

I am trying to pass a variable as a part of a bash command in my Rust program.

let speed_output = 70;
Command::new("nvidia-settings")
        .arg("-a")
        .arg("GPUTargetFanSpeed[fan:0]=speed_output")
        .spawn()
        .expect("nvidia-settings command failed to execute");

The program compiles correctly but my variable speed_output isn't passed to the bash shell. I would like to know how to pass variables from the Rust environment directly into the argument.