$\displaystyle{\left({x}+{2}\right)}{\left({x}+{10}\right)}={{x}}^{{2}}+{10}{x}+{2}{x}+{20}={{x}}^{{2}}+{12}{x}+{20}\usepackage{cmbright}\Huge\$ , not $\displaystyle{{x}}^{{2}}+{9}{x}+{20}\usepackage{cmbright}\Huge\$ . You first need to find two numbers whose product is 20 and whose sum is 9.