Use the power property: $\displaystyle{\log}_{{b}}{{M}}^{{n}}={n}\cdot{\log}_{{b}}{M}\usepackage{cmbright}\Huge\$

This simplifies $\displaystyle{\log}_{{{16}}}{{256}}^{{x}}\usepackage{cmbright}\Huge\$ to $\displaystyle{x}\cdot{\log}_{{{16}}}{256}\usepackage{cmbright}\Huge\$ .

Because $\displaystyle{256}={{16}}^{{2}}\usepackage{cmbright}\Huge\$ , $\displaystyle{\log}_{{{16}}}{256}\usepackage{cmbright}\Huge\$ simplifies to 2. So $\displaystyle{\log}_{{{16}}}{{256}}^{{x}}={x}\cdot{2}={2}{x}\usepackage{cmbright}\Huge\$ .