The Pythagorean Theorem is a^2 + b^2 = c^2
It’s usually used on triangles when two sides are given but one isn’t. C represents the hypotenuse while A and B are interchangeable. So solving for the hypotenuse would look like √a^2+ b^2= c but solving for a or be would look like √c^2-b^2 = a and √c^2-a^2= b.
So number wise √5^2+12^2 = 13