Pythagorean's Theorem exploits very important characteristics of a right triangle, and you should be familiar with this formula. Visually, given a right triangle, with sides x, y, and z:
the following relationships is derived:
x2 + y2 = z2
Note that z is the hypotenuse of the triangle. This is a very handy relationship, as it allows you to determine the length of a third side of a triangle given only two sides of the right triangle. The Pythagorean Theorem is used widely on the GRE, so you should definitely be familiar with it.