Correct.

$\displaystyle{y}={\log}_{{{5}}}{625}\usepackage{cmbright}\Huge\$ is equivalent to $\displaystyle{{5}}^{{y}}={625}\usepackage{cmbright}\Huge\$ .

Try different powers of 5: $\displaystyle{{5}}^{{2}}={25}\usepackage{cmbright}\Huge\$ , $\displaystyle{{5}}^{{3}}={125}\usepackage{cmbright}\Huge\$ , $\displaystyle{{5}}^{{4}}={625}\usepackage{cmbright}\Huge\$

So $\displaystyle{y}={4}\usepackage{cmbright}\Huge\$ .