Answer
$V=4a(a+b)(a-b)$
Work Step by Step
The larger rectangular prism has volume=(base area)$\times$height=
$V_{1}=a^{2}(4a)=4a^{3}$
the inner rectangular prism has volume
$V_{2}=b^{2}(4a)=4ab^{2}$
The volume we are after is
$V=V_{1}-V_{2}=4a^{3}-4ab^{2}$
$=4a(a^{2}-b^{2})\qquad$... a difference of squares,
$V=4a(a+b)(a-b)$
