This is not a factor of either monomial, because $\displaystyle{{m}}^{{6}}\usepackage{cmbright}\Huge\$ is not a factor of $\displaystyle{28}{{m}}^{{2}}{n}\usepackage{cmbright}\Huge\$ and n is not a factor of $\displaystyle{42}{{m}}^{{6}}\usepackage{cmbright}\Huge\$ . Find each prime they have in common and each variable they have in common. Make sure that you count how many times they have that factor in common. The GCF should have that many. The correct answer is $\displaystyle{14}{{m}}^{{2}}\usepackage{cmbright}\Huge\$ .