Trying to calculate the angle between two points in a circle?

First of all, let me say that I am absolutely brutal in mathematics, please bear with me.

I am trying to calculate the angle between two points on a circle, with two points being the distance the user has dragged for a certain amount of time.

This is what I still have:

intervalId = setInterval(function(){

      if(p1x != undefined){
        p2x = Xpos;

      }

      if(p1y != undefined){
        p2y = Ypos;
      }


      if( p1x != p2x || p1y != p2y ){

        p1a = p1x - wheelMiddleVer;
        p1b = p1y - wheelMiddleHor;
        a = Math.sqrt((p1a * p1a) + (p1b * p1b));

        p2a = p2x - wheelMiddleVer;
        p2b = p2y - wheelMiddleHor;
        b = Math.sqrt((p2a * p2a) + (p2b * p2b));

        u = p1x - p2x;
        v = p1y - p2y;
        c = Math.sqrt((u * u) + (v * v));


      }

      p1x = Xpos;
      p1y = Ypos;


  }, 1000);

I am not sure how to finish it. I tried to use the formula cos A = (b ^ 2 + c ^ 2 - a ^ 2) / 2bc, but this did not work for me. I will appreciate your contribution to this. If I can make the question clearer, let me know.