When the denominator of $\displaystyle\sqrt{{\frac{{{45}{x}}}{{{5}{y}}}}}\usepackage{cmbright}\Huge\$ is rationalized and the expression is simplified, the result is: $\displaystyle\frac{{?\sqrt{{{x}{y}}}}}{{y}}\usepackage{cmbright}\Huge\$

The question mark stands for _____.