How do I do the Pythagorean theory

you use the formula A² + B² = C². C is the longest side of the triangle. The other two dont really matter where you place them, in either A or B. 

 

In case you dont know how to solve it, ill give you an example:

A² + B² = C²

2² + 4² = C²   substitute 

4 + 16 = C²     simplify

20 = C²        add

4.472 = C     square both sides