It appears that you distributed $\displaystyle{9}{{m}}^{{3}}{{n}}^{{4}}\usepackage{cmbright}\Huge\$ only to the first term. You must distribute that factor to all terms in the parentheses. The correct answer is $\displaystyle{27}{{m}}^{{5}}{{n}}^{{4}}-{9}{{m}}^{{5}}{{n}}^{{6}}+{54}{{m}}^{{3}}{{n}}^{{6}}\usepackage{cmbright}\Huge\$ .