just for fun, i will do it differently
big rectangular box (including the missing portion) in lower right
= length x width x height
= 3 x 3 x 4
= 36ft^3
"missing" box at lower right = 1 x 3 x 2
= 6ft^3
so the ans = big box volume - missing box volume
= 36 - 6
= 30ft^3
