It appears that you incorrectly combined $\displaystyle-{3}{m}{n}\usepackage{cmbright}\Huge\$ and $\displaystyle{14}{m}{n}\usepackage{cmbright}\Huge\$ to get $\displaystyle{11}{{m}}^{{2}}{{n}}^{{2}}\usepackage{cmbright}\Huge\$ , and then combined that with the other $\displaystyle{{m}}^{{2}}{{n}}^{{2}}\usepackage{cmbright}\Huge\$ terms. When you combine like terms, the exponents do not change.