Scrolller scrolls the wrong way

1 
var center = Stage.width/2;
var speed = .05;
function activateMenu() {
 menu.onEnterFrame = function() {
  var distance = _root._xmouse-center;
  if (mask.hitTest(_root._xmouse, _root._ymouse)) {
   this._x += (distance*speed);
   if (this._x<-sub_menu._width/2) {
    this._x = 0;
   }
   if (this._x>0) {
    this._x = -sub_menu._width/2;
   }
  }
 };
}