Answer
$B=\dfrac{3V}{h}$
Work Step by Step
Using the properties of equality, the given equation, $
V=\dfrac{1}{3}Bh
,$ is equivalent to
\begin{array}{l}\require{cancel}
3V=Bh
\\\\
\dfrac{3V}{h}=\dfrac{Bh}{h}
\\\\
B=\dfrac{3V}{h}
.\end{array}