Which property is used in the following three times 5 + 9 equal 3 * 5 + 3 * 9 destructive property none of the above commutative property or associative property ​

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