You were probably thinking about $\displaystyle{{f}}^{{{8}+{5}}}\usepackage{cmbright}\Huge\$ , which could be rewritten as $\displaystyle{{f}}^{{13}}\usepackage{cmbright}\Huge\$ . Any radical in the form $\displaystyle{\sqrt[{{n}}]{{{{f}}^{{x}}}}}\usepackage{cmbright}\Huge\$ can be rewritten as $\displaystyle{{f}}^{{\frac{{x}}{{n}}}}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{{f}}^{{\frac{{8}}{{5}}}}\usepackage{cmbright}\Huge\$ .