int x,y;
int direction;
float dx,dy;
float distance;

void setup(){

  size(300,300);

  direction = 1;

}

void loop(){

  //------CALCULATE DISTANCE------
  dx=mouseX-x;
  dy=mouseY-y;
  distance=sqrt(dx*dx+dy*dy);

  background(255,255,255);
  line(0,mouseX,300,mouseY);
  x += direction;
  y += direction;
  dx += direction;
  dy += direction;
  

  if(x>300 || x<0){
    direction = direction * -1;
  }
}