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\$ .

$\displaystyle{16}{{x}}^{{2}}+{72}{x}+{81}={{\left({4}{x}\right)}}^{{2}}+{2}{\left({4}{x}\right)}{\left({9}\right)}+{{9}}^{{2}}={{\left({4}{x}+{9}\right)}}^{{2}}\usepackage{cmbright}\Huge\$