You may have been thinking about factoring a perfect square trinomial, but the given expression is a difference of squares. This can be factored using $\displaystyle{{r}}^{{2}}-{{s}}^{{2}}={\left({r}+{s}\right)}{\left({r}-{s}\right)}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{\left({9}{x}+{3}\right)}{\left({9}{x}-{3}\right)}\usepackage{cmbright}\Huge\$ .