Pulling out cubed terms and simplifying the expression, you find that $\displaystyle{\sqrt[{{3}}]{{-{108}{{a}}^{{16}}{{b}}^{{14}}{{c}}^{{7}}}}}=-{3}{{a}}^{{5}}{{b}}^{{4}}{{c}}^{{2}}{\sqrt[{{3}}]{{{4}{a}{{b}}^{{2}}{c}}}}\usepackage{cmbright}\Huge\$ .