You may have correctly rewritten the radical as $\displaystyle{{\left({9}{{m}}^{{2}}{{n}}^{{16}}\right)}}^{{\frac{{1}}{{4}}}}\usepackage{cmbright}\Huge\$ , but you did not use the laws of exponents to simplify. You applied the exponent $\displaystyle\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ only to the last factor, $\displaystyle{{n}}^{{16}}\usepackage{cmbright}\Huge\$ . Instead, you should rewrite this expression as $\displaystyle{{9}}^{{\frac{{1}}{{4}}}}\cdot{{m}}^{{\frac{{2}}{{4}}}}\cdot{{n}}^{{\frac{{16}}{{4}}}}\usepackage{cmbright}\Huge\$ , rewrite 9 in exponential form, and simplify the exponents.