[MX] cant make function work!

programming flash
1

I was reading Kirupa Chinnathambi’s function tutorial and I was trying to make my own function, but it’s not working! Here it is:

move = function (finalX, finalY, k) { 
	cX = this._x;
	cY = this._y;
	difX = cX-finalX;
	difY = cY-finalY;
  	setProperty(this, _x, cX-(difX/k));
  	setProperty(this, _y, cY-(difY/k));
}

_root.btn.onRelease = function() {
	_root.ball.move ("200", "200", "5");
	}

thanks for the help!