We can calculate the volume a large box (that includes the empty space) by:
Vwhole=30(24+9)(6)
Then we can calculate the volume of the empty space:
Vempty=24(30-9)(6)
From that, we can subtract the empty space by doing
(30(24+9)(6))-(24(30-9)(6))
Which equals 2916cm^3.
