Factor by grouping. Then supply the term that is missing below.

$\displaystyle{3}{m}{n}+{2}{m}+{12}{n}+{8}={\left({m}+{4}\right)}{\left(?+{2}\right)}\usepackage{cmbright}\Huge\$

The missing term $\displaystyle?=\usepackage{cmbright}\Huge\$ _____.