Hi there! The formula for hypotenuse is a^2 + b^2 = c^2. Side a = 12, side b = 9, and we are looking for the value of c. 12^2 is 144. 9^2 is 81. 144 + 81 is 225. Now, find the square root of 225 to find the value of c. The square root if 225 is 15. There. The hypotenuse is 15 ft.