1. Calculate the volume of entire cube as if it was a solid rectangle:
Length x width x height=
20x16x12 = 3,940 cubic inches
2. Calculate the volume of the "missing"
Piece and subtract:
Length x width x height=
20x8x6 = 960 cubic inches
(notice you can find the height of the missing piece by subtracting 12-6)
So your answer is 3,940-960 = 2,980 cubic inches inside the steps