You forgot to use a grouping symbol to show that the entire expression under the radical will be raised to the one-eighth power. This answer is equivalent to $\displaystyle{256}{\sqrt[{{8}}]{{{z}}}}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{{\left({256}{z}\right)}}^{{\frac{{1}}{{8}}}}\usepackage{cmbright}\Huge\$ .