Hi!
The equation to find the surface are of a rectangular prism is
(a = area
h = height
w = width
l = length)
a = 2(wl + wh + hl)
Put the values into the equation, and solve.
a = 2(5 · 9 + 5 · 7.5 + 9 · 7.5)
a = 2(45 + 37.5 + 67.5)
a = 2 · 150
a = 300
The surface area is 300ft.
Hope this helps!
-Peredhel