You incorrectly used the index of the radical as the exponent in the expression. The rule to convert a radical is: $\displaystyle{\sqrt[{{n}}]{{{x}}}}={{x}}^{{\frac{{1}}{{n}}}}\usepackage{cmbright}\Huge\$

Because the index is 8, $\displaystyle\frac{{1}}{{8}}\usepackage{cmbright}\Huge\$ will be the rational exponent. The correct answer is $\displaystyle{{\left({256}{z}\right)}}^{{\frac{{1}}{{8}}}}\usepackage{cmbright}\Huge\$ .