The diagonals are perpendicular to each other. The right triangle has half of each diagonal as sides, so the hypotenuse or side length is Sqrt(9^2 + 12^2) = 15. Hint: a 3,4,5 sided triangle is the smallest integer right triangle. Doubling or tripling, etc. is still a right triangle. So when I saw the 9,12, I knew that 15 was the hypotenuse.
