You have the following expression:
[tex]A=3h[/tex]In order to solve for h, use the division property of equality. In this case divide by 3 both sides:
[tex]\begin{gathered} \frac{A}{3}=\frac{3h}{3} \\ \frac{A}{3}=h \end{gathered}[/tex]Hence, the solution for h = A/3