[tex]\bf \textit{height of an equilateral triangle}\\\\
h=\cfrac{s\sqrt{3}}{2}\qquad
\begin{cases}
s=length~of\\
\qquad a~side\\
------\\
s=12
\end{cases}\implies h=\cfrac{12\sqrt{3}}{2}\implies h=6\sqrt{3}\\\\
-------------------------------\\\\
\textit{area of an equilateral triangle}\\\\
A=\cfrac{s^2\sqrt{3}}{4}\qquad
\begin{cases}
s=length~of\\
\qquad a~side\\
------\\
s=12
\end{cases}\implies A=\cfrac{12^2\sqrt{3}}{4}\implies A=36\sqrt{3}[/tex]
and of course as you already know the volume of the prism is just the area of the base times the length, namely A * 20.
