You may have correctly multiplied both sides by 3 as the first step. However, to isolate h in the equation $\displaystyle{3}{V}=\pi{{r}}^{{2}}{h}\usepackage{cmbright}\Huge\$ , you need to divide both sides by $\displaystyle\pi{{r}}^{{2}}\usepackage{cmbright}\Huge\$ , not subtract it.