Check the picture below.
[tex]\bf \textit{volume of a pyramid}\\\\ V=\cfrac{1}{3}Bh~~ \begin{cases} B=&area~of\\ &its~base\\ h=&height\\ \cline{1-2} B=&\frac{1}{2}(3)(4)\\ h=&10 \end{cases}\implies V=\cfrac{1}{3}\cdot \cfrac{1}{2}(3)(4)(10)\implies V=20[/tex]