Well you should know first what the pythagorean theorem is. then you should memorize it in order for you to solve `something that needed it. 
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b the way ou dont solve a pythagorean theorem :) it is actually a great way to solve
Just follow the formula which is c²=a²+b²
for ex the hypotenuse is c then the opposite is b and the adjacent is a so the formula will be look like this c=√a²+b²