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