\[b = 8\ дм:\ \ \]
\[a = b + 4\ дм = 8 + 4 =\]
\[= 12\ (дм) - длина.\]
\[h = a\ \ :3 = 12\ :3 = 4\ (дм) -\]
\[высота.\]
\[V = abh = 8 \cdot 12 \cdot 4 = 32 \cdot 12 =\]
\[= \ 384\ \left( дм^{3} \right).\]
\[Ответ:384\ дм^{3}.\]