It appears that you used FOIL, but multiplied instead of adding exponents for each product. The correct answer is $\displaystyle{45}{{m}}^{{6}}-{30}{{m}}^{{4}}{{n}}^{{2}}-{54}{{m}}^{{2}}{{n}}^{{4}}+{36}{{n}}^{{6}}\usepackage{cmbright}\Huge\$ .