$\displaystyle{\left({x}+{5}\right)}{\left({x}-{8}\right)}={{x}}^{{2}}-{8}{x}+{5}{x}+{40}={{x}}^{{2}}-{3}{x}+{40}\usepackage{cmbright}\Huge\$ , not $\displaystyle{{x}}^{{2}}+{3}{x}-{40}\usepackage{cmbright}\Huge\$ . You first need to find two numbers whose product is $\displaystyle-{40}\usepackage{cmbright}\Huge\$ and whose sum is 3. The correct answer is $\displaystyle{\left({x}-{5}\right)}{\left({x}+{8}\right)}\usepackage{cmbright}\Huge\$ .