When determining if three given lengths could represent the sides of a right triangle, how do you choose which side length to put on the side of the
Pythagorean Theorem that has only one variable?

Yeah what she said right

Use pythagorean’s theorem and find the missing side lengths of the two smaller triangles and add them together
13^2 - 12^2 = 5
15^2 - 12^2 = 9
5+9 = 14
14 is the length of each side of the square
14^2 + 14^2 = 19.8
19.8 is the diagonal length of the inside if the square