If you're stuck, take a look at a point on the preimage and a corresponding point on the image (say, A and A'). How far did it move, and in what direction? Is this the same for all of the points?
Two shapes are congruent if they are identical in every measurement, though they don't have to be in the same position or orientation. Did the shape change from preimage to image? If not, then the two are congruent.
