Correct. In general, $\displaystyle{{r}}^{{2}}-{{s}}^{{2}}={\left({r}-{s}\right)}{\left({r}+{s}\right)}\usepackage{cmbright}\Huge\$ , so $\displaystyle{{x}}^{{2}}-{81}={{x}}^{{2}}-{{9}}^{{2}}={\left({x}-{9}\right)}{\left({x}+{9}\right)}\usepackage{cmbright}\Huge\$ .