To find the product, you can use $\displaystyle{\left({a}-{b}\right)}{\left({a}+{b}\right)}={{a}}^{{2}}-{{b}}^{{2}}\usepackage{cmbright}\Huge\$ . In this case $\displaystyle{a}={3}{x}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={1}\usepackage{cmbright}\Huge\$ , so $\displaystyle{\left({3}{x}+{1}\right)}{\left({3}{x}-{1}\right)}={{\left({3}{x}\right)}}^{{2}}-{{1}}^{{2}}={9}{{x}}^{{2}}-{1}\usepackage{cmbright}\Huge\$ .