so the pyramid looks like the one in the picture below.
notice, the bottom of the pyramid is a square, thus is called that, and each side is 3 units long, thus
[tex]\bf \textit{volume of a pyramid}\\\\
V=\cfrac{1}{3}Bh~~
\begin{cases}
B=area~of\\
\qquad its~base\\
h=height\\
----------\\
V=12\\
B=\stackrel{3\times 3}{9}
\end{cases}\implies 12=\cfrac{(9)h}{3}\implies 12=3h
\\\\\\
\cfrac{12}{3}=h\implies 4=h[/tex]
