$\displaystyle{\left({x}+{7}\right)}{\left({x}+{10}\right)}={{x}}^{{2}}+{10}{x}+{7}{x}+{70}={{x}}^{{2}}+{17}{x}+{70}\usepackage{cmbright}\Huge\$ , not $\displaystyle{{x}}^{{2}}+{17}{x}+{30}\usepackage{cmbright}\Huge\$ . You first need to find two numbers whose product is 30 and whose sum is 17.