controlling _mc

I have an image of a van which now moves to the side when the user hovers over. I have a piece of text saved as a movie clip called text_mc.
I want the text to be hidden until the image moves and then to appear. I have used the _alpha command but this doesn't seem to sort it. Am i missing something obvious-still a bit new to this actionscript malarky!
cheers


text_mc._alpha = 0;
myButton_btn.onRollOver = function() {
tweenBall(mx.transitions.easing.easeOut);
};
function tweenBall(easeType) {
var begin = 20;
var end = 800;
var time = 50;
var mc = ball_mc;
ballTween = new mx.transitions.Tween(mc, "_x", easeType, begin, end, time);
}
text_mc._alpha =100;