Use the quotient property: $\displaystyle{\log}_{{b}}\frac{{M}}{{N}}={\log}_{{b}}{M}-{\log}_{{b}}{N}\usepackage{cmbright}\Huge\$

Then use the fact that $\displaystyle{{5}}^{{4}}={625}\usepackage{cmbright}\Huge\$ .

$\displaystyle{\log}_{{{5}}}{\left(\frac{{625}}{{x}}\right)}={\log}_{{{5}}}{625}-{\log}_{{{5}}}{x}={4}-{\log}_{{5}}{x}\usepackage{cmbright}\Huge\$