Explanation:
this is a combined object :
there is the basic block at the bottom (21×21×17), and there is a pyramid on top of it.
so, we need to calculate the volume of both parts and add them for the total volume.
the volume of the block is, as mentioned :
21 × 21 × 17 (length×width×height) = 7,497 ft³
the volume of a pyramid is
length×width×height/3
which is in our case
21 × 21 × 9 / 3 = 21 × 21 × 3 = 1,323 ft³
so, the total volume is
7,497 + 1,323 = 8,820 ft³