You found the solution to $\displaystyle{2}={\log}_{{b}}{625}\usepackage{cmbright}\Huge\$ . Remember that $\displaystyle{4}={\log}_{{{b}}}{625}\usepackage{cmbright}\Huge\$ is equivalent to $\displaystyle{{b}}^{{4}}={625}\usepackage{cmbright}\Huge\$ . Try different bases until you find a number to the 4th power equal to 625.