You may have reversed the signs of the exponents. This answer would be correct if the given expression were $\displaystyle{{x}}^{{-{{4}}}}\cdot{{x}}^{{2}}\usepackage{cmbright}\Huge\$ . The actual expression is equal to $\displaystyle{{5}}^{{4}}\cdot{{5}}^{{-{{2}}}}\usepackage{cmbright}\Huge\$ .