Position-problem

Well, this code

import mx.transitions.Tween;
import mx.transitions.easing.*;

function movon (_tox) {
var move1:Tween = new Tween (indhold_mc, “_x”, Strong.easeOut, indhold_mc._x, _tox, 2, true);

}

right.onRelease = function () {
movon (213.5 );
};
left.onRelease = function () {
movon (237.5);
};

I used it from at tutorial. How do i do, so when i press the buttons they dont go to chosen position, but +/- to the position the movieclip has at the time?

Thanks in advance!

-JPAG