Chapter 5 - Section 5.5 - Factoring Special Forms - Exercise Set - Page 373: 111

$3a(a+b)(a-b)$

The product of a binomial sum and a binomial difference is: $(A+B)(A-B)=A^2-B^2$. The square of a binomial sum is: $(A+B)^2=A^2+2AB+B^2$. The square of a binomial difference is: $(A-B)^2=A^2-2AB+B^2$. Hence here by deducing the volume of the smaller one from the big one: $V=a\cdot a\cdot3a-b\cdot b\cdot3a=3a^3-3ab^2=3a(a^2-b^2)=3a(a+b)(a-b)$