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

This simplifies $\displaystyle{\log}_{{{7}}}{{7}}^{{x}}\usepackage{cmbright}\Huge\$ to $\displaystyle{x}\cdot{\log}_{{{7}}}{7}\usepackage{cmbright}\Huge\$ .

Because $\displaystyle{7}={{7}}^{{1}}\usepackage{cmbright}\Huge\$ , $\displaystyle{\log}_{{{7}}}{7}\usepackage{cmbright}\Huge\$ simplifies to 1. So $\displaystyle{\log}_{{{7}}}{{7}}^{{x}}={x}\cdot{1}={x}\usepackage{cmbright}\Huge\$ .