Correct. Use the quotient property, followed by the power property: $\displaystyle{\log}_{{{5}}}\frac{{y}}{{{{125}}^{{x}}}}={\log}_{{{5}}}{y}-{\log}_{{{5}}}{\left({{125}}^{{x}}\right)}={\log}_{{{5}}}{y}-{x}{\log}_{{{5}}}{125}={\log}_{{{5}}}{y}-{x}{\left({3}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle={\log}_{{{5}}}{y}-{3}{x}\usepackage{cmbright}\Huge\$

The question mark stands for $\displaystyle{3}{x}\usepackage{cmbright}\Huge\$ .