one way is to find the volume of the whole then find the volume of the cut out part then minus cut out from total
aka
Vprism=Vtotal-Vcutout
Vtotal=Legnth times width times height
Vtotal=20 times 10 times 15
Vtotal=3000 in^3
cutout is 10 by 7 by 8
Vcutout=10*7*8
Vcutout=560 in^3
Vprism=Vtotal-Vcutout
Vprism=3000in^3-560 IN^3
Vprism=2440in^3
volume=2440 in^3, or 2440 cubic inches