$\displaystyle{81}{{x}}^{{2}}={{\left({9}{x}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ and $\displaystyle{9}={{3}}^{{2}}\usepackage{cmbright}\Huge\$ , so this a difference of squares. Use $\displaystyle{{r}}^{{2}}-{{s}}^{{2}}={\left({r}+{s}\right)}{\left({r}-{s}\right)}\usepackage{cmbright}\Huge\$ to get $\displaystyle{81}{{x}}^{{2}}-{9}={\left({9}{x}+{3}\right)}{\left({9}{x}-{3}\right)}\usepackage{cmbright}\Huge\$ .