You didn't show your attempt at the problem but I can guess that you have no idea how to start so I'll give you a tip.
1 cubic foot is equal to 1 foot X 1 foot x 1 foot.
However, 8 ft.3 is equal to 2 ft. X 2 ft. X 2 ft.
So, 8 ft.3 is equal to 24 in. X 24 in. X 24 in.
Hope it helps.