You have correctly combined the $\displaystyle{{x}}^{{2}}\usepackage{cmbright}\Huge\$ terms and the $\displaystyle{{y}}^{{2}}\usepackage{cmbright}\Huge\$ terms. However, you have made a mistake with the $\displaystyle{x}{y}\usepackage{cmbright}\Huge\$ terms. When you combine like terms, their exponents do not change.