Hittest REALLY BASIC QUESTION!

I am just starting to learn Flash and am wondering how to do one thing in particular. I want to have bars move across the screen from left to right. Nothing advanced, just left to right continuously. I have tried modifying different tutorials but cant seem to figure out how to get the thing to move back to the left when it gets to a certain point on the stage. I tried using hitTest:
onClipEvent (enterFrame) {
speed = 2;
this._x += speed;
if (this, hitTest(_root.block)) {
this._x = -2;
}
}

But this doesnt work. It just makes the thing jump left, not reversse directions. Thanks for any help.