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

The denominator of the fraction is 4, so the radical will be for a fourth root. Because there are parentheses, the exponent refers to the quantity within the parentheses.

$\displaystyle{{\left({5}{y}\right)}}^{{\frac{{3}}{{4}}}}={\sqrt[{{4}}]{{{{\left({5}{y}\right)}}^{{3}}}}}\usepackage{cmbright}\Huge\$