Because the denominators are the same, you can just subtract the numerators:

$\displaystyle\frac{{{9}{x}}}{{{x}+{4}}}-\frac{{{4}{x}-{20}}}{{{x}+{4}}}=\frac{{{9}{x}-{\left({4}{x}-{20}\right)}}}{{{x}+{4}}}=\frac{{{9}{x}-{4}{x}+{20}}}{{{x}+{4}}}=\frac{{{5}{x}+{20}}}{{{x}+{4}}}\usepackage{cmbright}\Huge\$

$\displaystyle=\frac{{{5}{\left({x}+{4}\right)}}}{{{x}+{4}}}={5}\usepackage{cmbright}\Huge\$