The rule for rewriting rational exponents is: $\displaystyle{{x}}^{{\frac{{1}}{{n}}}}={\sqrt[{{n}}]{{{x}}}}\usepackage{cmbright}\Huge\$

The denominator of the fraction is 3, so this denominator becomes a cube root.

$\displaystyle{{64}}^{{\frac{{1}}{{3}}}}={\sqrt[{{3}}]{{{64}}}}\usepackage{cmbright}\Huge\$