Simplify the expression $\displaystyle\frac{{{{x}}^{{2}}+{3}{x}-{18}}}{{{{x}}^{{3}}+{8}{{x}}^{{2}}+{12}{x}}}\usepackage{cmbright}\Huge\$ and then supply the missing number.

$\displaystyle\frac{{{{x}}^{{2}}+{3}{x}-{18}}}{{{{x}}^{{3}}+{8}{{x}}^{{2}}+{12}{x}}}=\frac{{{x}-{3}}}{{{{x}}^{{2}}+?{x}}}\usepackage{cmbright}\Huge\$

The question mark stands for the number _____.