Correct. First notice that $\displaystyle{{81}}^{{\frac{{1}}{{4}}}}={3}\usepackage{cmbright}\Huge\$ , since $\displaystyle{{3}}^{{4}}={81}\usepackage{cmbright}\Huge\$ . Therefore, $\displaystyle{{81}}^{{\frac{{3}}{{4}}}}={{\left({{81}}^{{\frac{{1}}{{4}}}}\right)}}^{{3}}={{3}}^{{3}}={27}\usepackage{cmbright}\Huge\$ .