Answer:
The Pythagorean theorem is a^2+b^2=c^2 where c is the length of the longest, diagonal side (diagonal means opposite the right angle/small square).
Explanation:
If you're looking for a short side (i.e. next to the right angle), square the longest one and subtract the square of the other known.
If you're looking for a long side (i.e. opposite the right angle), square both you know and add them together.
In both cases, take the Square Root with your calculator, or by trial and error, find a number that squares to make your current number.
[1] A square of the number x is x times x.