You probably tried to subtract numerators, but you forgot to distribute the minus sign to the $\displaystyle-{20}\usepackage{cmbright}\Huge\$ . The first step is:

$\displaystyle\frac{{{9}{x}}}{{{x}+{4}}}-\frac{{{4}{x}-{20}}}{{{x}+{4}}}=\frac{{{9}{x}-{\left({4}{x}-{20}\right)}}}{{{x}+{4}}}\usepackage{cmbright}\Huge\$