Check the picture below.
so we can simply get the volume of each and sum them up.
[tex]\textit{volume of a pyramid}\\\\ V=\cfrac{Bh}{3}~~ \begin{cases} h=height\\ B=base's~area\\[-0.5em] \hrulefill\\ h=6\\ B=\stackrel{8\times 8}{64} \end{cases}\implies V=\cfrac{(64)(6)}{3}\implies V=128[/tex]
[tex]\textit{volume of a pyramid}\\\\ V=\cfrac{Bh}{3}~~ \begin{cases} h=height\\ B=base's~area\\[-0.5em] \hrulefill\\ h=12\\ B=\stackrel{8\times 8}{64} \end{cases}\implies V=\cfrac{(64)(12)}{3}\implies V=256 \\\\[-0.35em] ~\dotfill\\\\ ~\hfill 128~~ + ~~256~~ = ~~384~\hfill[/tex]
