How to make objects move left to right in a screen upon hit test object

I want to make a symbol 'enemy_mc' move left and right by 20 units upon hitting hittestobject, as in when my player model, head_mc tocuhes a symbol, wall_mc, then objects, (the enemies) will move left to right on the screen.The enemies are already in place on the map, hit test object will activate their movement. The map size is 550 by 400.

Thanks