Use the quotient property, $\displaystyle{\log}_{{b}}\frac{{M}}{{N}}={\log}_{{b}}{M}-{\log}_{{b}}{N}\usepackage{cmbright}\Huge\$ , and then simplify, using the fact that $\displaystyle{10}={{10}}^{{1}}\usepackage{cmbright}\Huge\$ .

$\displaystyle{\log}_{{{10}}}{\left(\frac{{x}}{{{10}}}\right)}={\log}_{{{10}}}{x}-{\log}_{{{10}}}{10}={\log}_{{{10}}}{x}-{1}\usepackage{cmbright}\Huge\$