You may have found the correct value of k, by using $\displaystyle{V}={k}{{r}}^{{2}}{h}\usepackage{cmbright}\Huge\$ and solving $\displaystyle{251.2}={k}{\left({{4}}^{{2}}\right)}{\left({5}\right)}\usepackage{cmbright}\Huge\$ , but you incorrectly solved the second equation by not squaring the radius r, which is 3.