1. create a line across the stage, then insert keyframe in frame 10
turn layer into guide layer.
2.under the guide layer create a new layer, turn this layer into the guided layer.
create an object to motion tween, create a motion tween with this object, insert key frame in frame 10.
3. take motion tweened object and put the first frame of motion tween on one end of your line on the guide layer.
put the last frame of the motion tweened object and place in on the opposite end of the line.