Let y = the distance between the top left corner and the bottom right corner
y^2 = 24^2 + 15^2
y^2 = 801
y = 3 * sqrt(89)
Now we can find x.
12^2 + x^2 = (3 * sqrt(89))^2
x^2 = 657
x = 3 * sqrt(73) or 25.63