Explanation:
this creates a right-angled triangle, with the length of the slide being the Hypotenuse (the side opposite of the 90° angle), and height and base are the legs.
so, we can use Pythagoras
c² = a² + b²
with c being the Hypotenuse, a and b being the legs.
length² = 9² + 12² = 81 + 144 = 225
length = sqrt(225) = 15 ft