The answer is 22.5 ft. To get this answer you use the equation V = lwh. You know volume is 472.5, the length is 7 and the width is 3. Plugged these values in and solve for h.
V = lwh
472.5 = (7)(3)(h)
472.5 = 21h (Divide by 21)
22.5 = h (The answer you are looking for.)
