giúp mình với ....

$\begin{array}{l}
{V_{ABMN}} = \dfrac{1}{3}{d_{\left( {M,\left( {ABN} \right)} \right)}}.{S_{ABN}}\\
 = \dfrac{1}{3}.\dfrac{1}{2}{d_{\left( {A,\left( {ABN} \right)} \right)}}.\dfrac{1}{2}{S_{ABC}}\\
 = \dfrac{1}{4}.\dfrac{1}{3}{d_{\left( {A,\left( {ABN} \right)} \right)}}{S_{ABC}}\\
 = \dfrac{1}{4}{V_{S.ABC}} = \dfrac{1}{4}.24 = 6 \to A
\end{array}$