The formula for finding the volume of a rectangular prism (the cargo area of the truck is the shape of a rectangle in 3D; a rectangular prism) is V = (l)(w)(h); where l = length, w = width, and h = height.
First, substitute the known values into the equation:
l = 8.5
w = 6
h = 10.5
V = (8.5)(6)(10.5)
(Note: .5 = 1/2)
Now, all we need to do is simplify:
V = 535.5 ft³ OR 535 1/2 ft³
(Note: ft³ is the condensed form of cubic feet)
You can pick whichever form your test directs you to use. They are both the same value though.
Hope this helps!