Incorrect. $\displaystyle{{x}}^{{2}}-{81}={{x}}^{{2}}-{{9}}^{{2}}\usepackage{cmbright}\Huge\$ . This can be factored using $\displaystyle{{r}}^{{2}}-{{s}}^{{2}}={\left({r}-{s}\right)}{\left({r}+{s}\right)}\usepackage{cmbright}\Huge\$ .