You probably recognized that the first and last terms are perfect squares, but you used the wrong sign. If you multiply this answer out, you will get a middle term of $\displaystyle{40}{m}\usepackage{cmbright}\Huge\$ , not $\displaystyle-{40}{m}\usepackage{cmbright}\Huge\$ . A trinomial in the form $\displaystyle{{r}}^{{2}}-{2}{r}{s}+{{s}}^{{2}}\usepackage{cmbright}\Huge\$ can be factored as $\displaystyle{{\left({r}-{s}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ .