Correct. $\displaystyle{\left({a}+{b}\right)}{\left({a}-{b}\right)}={{a}}^{{2}}-{{b}}^{{2}}\usepackage{cmbright}\Huge\$ . In this case $\displaystyle{a}={2}{m}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={6}\usepackage{cmbright}\Huge\$ , so $\displaystyle{\left({2}{m}+{6}\right)}{\left({2}{m}-{6}\right)}={{\left({2}{m}\right)}}^{{2}}-{{6}}^{{2}}={4}{{m}}^{{2}}-{36}\usepackage{cmbright}\Huge\$ . The missing number is equal to 36.