Pythagorean Theorem is used to find the missing dimension of a side of any right triangle. This theorem does not work with any other triangles other than right triangles which have a perpendicular angle of 90°. The photo I have attached shows the formula for Pythagorean Theorem. c squared is the hypotenuse and you can switch around a and b to be any of the remaining sides other than the hypotenuse. The hypotenuse is the longest side of the right triangle. Using this formula, you can find any missing side by rearranging the letters.
For example c squared - a squared would give you b squared. This is a rearranged formula for finding a side other than the hypotenuse.