Incorrect. To simplify an expression, reorder the addends and combine like terms. $\displaystyle{9}{{m}}^{{2}}{{n}}^{{2}}\usepackage{cmbright}\Huge\$ and $\displaystyle-{4}{{m}}^{{2}}{{n}}^{{2}}\usepackage{cmbright}\Huge\$ are like terms. The correct answer is 5.