You can't go very far without knowing this basic theorem. The Pythagorean Theorem relates right angle triangles and squares. If you search for the Pythagorean Theorem pictures, you will see a right triangle with squares on each of it's feet. The smallest square + the middle sized square = the largest square. Notice that word plus. It means that the squares are added and that is the key word in answering your question.
The answer is B.
I seem not to be able to download a picture. I hope you have better luck.
