Use FOIL to multiply two binomials.

FIRST: $\displaystyle{5}{{m}}^{{4}}\cdot{9}{{m}}^{{2}}={45}{{m}}^{{6}}\usepackage{cmbright}\Huge\$

OUTER: $\displaystyle{5}{{m}}^{{4}}\cdot{\left(-{6}{{n}}^{{2}}\right)}=-{30}{{m}}^{{4}}{{n}}^{{2}}\usepackage{cmbright}\Huge\$ INNER: $\displaystyle-{6}{{n}}^{{4}}\cdot{9}{{m}}^{{2}}=-{54}{{m}}^{{2}}{{n}}^{{4}}\usepackage{cmbright}\Huge\$

LAST: $\displaystyle-{6}{{n}}^{{4}}\cdot{\left(-{6}{{n}}^{{2}}\right)}={36}{{n}}^{{6}}\usepackage{cmbright}\Huge\$

Add. Combine like terms, if any: $\displaystyle{45}{{m}}^{{6}}-{30}{{m}}^{{4}}{{n}}^{{2}}-{54}{{m}}^{{2}}{{n}}^{{4}}+{36}{{n}}^{{6}}\usepackage{cmbright}\Huge\$