Note that the radical is a fifth root, not a cube root. What is the fifth root of $\displaystyle{{a}}^{{7}}\usepackage{cmbright}\Huge\$ ? What about $\displaystyle{{b}}^{{12}}\usepackage{cmbright}\Huge\$ and $\displaystyle{{c}}^{{11}}\usepackage{cmbright}\Huge\$ ?