A student evaluates $\displaystyle{{x}}^{{2}}-{2}{y}\usepackage{cmbright}\Huge\$ when $\displaystyle{x}=-{3}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}={4}\usepackage{cmbright}\Huge\$ and gets the answer $\displaystyle-{17}\usepackage{cmbright}\Huge\$ . If the answer is not correct, which is the most plausible error that would lead to the answer that the student obtained?