$\displaystyle{20}={1}\cdot{20}={2}\cdot{10}={4}\cdot{5}\usepackage{cmbright}\Huge\$ . The factors whose sum is 9 are 4 and 5. Now rewrite the trinomial and use grouping:

$\displaystyle{{x}}^{{2}}+{4}{x}+{5}{x}+{20}={x}{\left({x}+{4}\right)}+{5}{\left({x}+{4}\right)}={\left({x}+{4}\right)}{\left({x}+{5}\right)}\usepackage{cmbright}\Huge\$