Look at the picture.
We have 3 cuboids. The formula of a volume of a cuboid is:
V = Length × Width × Height = lwh
The cuboid 1 are congruent to the cuboid 2.
1&2
l = 9 in, w = 10 in, h = 10 in
V = (9)(10)(10) = 900 in³
3
l = 6 in, w = 10 in, h = 10 in
V = (6)(10)(10) = 600 in³
The volume of the composite prism:
V = 2(900) + 600 = 1800 + 600 = 2400 in³