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{9}{{x}}^{{2}}+{42}{x}+{49}={{\left({3}{x}\right)}}^{{2}}+{2}{\left({3}{x}\right)}{\left({7}\right)}+{{7}}^{{2}}={{\left({3}{x}+{7}\right)}}^{{2}}\usepackage{cmbright}\Huge\$