Ian started to simplify the following logarithm:

$\displaystyle{\log}_{{5}}{y}{{x}}^{{3}}={\log}_{{{5}}}{y}+{\log}_{{{5}}}{{x}}^{{3}}={\log}_{{5}}{y}+?{\log}_{{5}}{x}\usepackage{cmbright}\Huge\$

The question mark stands for _____.