$\displaystyle-{40}={1}\cdot{\left(-{40}\right)}={2}\cdot{\left(-{20}\right)}={4}\cdot{\left(-{10}\right)}={5}\cdot{\left(-{8}\right)}=-{1}\cdot{40}=-{2}\cdot{20}=-{4}\cdot{10}=-{5}\cdot{8}\usepackage{cmbright}\Huge\$ . The factors whose sum is 3 are $\displaystyle-{5}\usepackage{cmbright}\Huge\$ and 8. Now rewrite the trinomial and use grouping:

$\displaystyle{{x}}^{{2}}-{5}{x}+{8}{x}-{40}={x}{\left({x}-{5}\right)}+{8}{\left({x}-{5}\right)}={\left({x}-{5}\right)}{\left({x}+{8}\right)}\usepackage{cmbright}\Huge\$