The Pythagorean Theory is a way to find the missing length of a right triangle. Here is a right triangle with a description.
The Formula for Pythagorean's Theory is A squared + B squared = C squared. If the hypotenuse is missing then you would use the formula above. If one of the legs is missing you would use C squared - B squared = A squared or C squared - A squared = B squared. You could use this if you were a carpenter because they always use right triangles. Here is a link to help you.