The student failed to square the negative sign when evaluting the $\displaystyle{{x}}^{{2}}\usepackage{cmbright}\Huge\$ part. This led to calculating $\displaystyle{{x}}^{{2}}\usepackage{cmbright}\Huge\$ as $\displaystyle-{9}\usepackage{cmbright}\Huge\$ instead of $\displaystyle{9}\usepackage{cmbright}\Huge\$ .