You may have simply swapped the variable $\displaystyle{h}\usepackage{cmbright}\Huge\$ and the variable $\displaystyle{V}\usepackage{cmbright}\Huge\$ when trying to solve for $\displaystyle{h}\usepackage{cmbright}\Huge\$ . To isolate the variable, you need to perform operations on both sides. For example, you can multiply both sides by 3 and you can divide both sides by $\displaystyle\pi{{r}}^{{2}}\usepackage{cmbright}\Huge\$ .