What sort of help? Do you need to prove it? Do you need help visualizing it?
If it is the latter, go down to the local hardware store and pick up a door hinge. Open and close the hinge. The flaps of the hinge are the congruent sides of the triangle mentioned in the theorem. The distance between the outer edges of the flaps is the third side of the triangle, and the angle between the flaps is the included angle that the theorem mentions. So as you open the hinge the angle gets bigger and the distance between the edges of the flaps gets greater -- and vice versa.