Rewrite the radical using a rational exponent. The root, 3, determines the denominator of the fraction, so $\displaystyle\frac{{1}}{{3}}\usepackage{cmbright}\Huge\$ will be the rational exponent. Use a grouping symbol to show that the entire expression under the radical will be raised to the one-third power.

$\displaystyle{\sqrt[{{3}}]{{{12}{{m}}^{{2}}}}}={{\left({12}{{m}}^{{2}}\right)}}^{{\frac{{1}}{{3}}}}\usepackage{cmbright}\Huge\$