You switched the values of x and y. That is, when $\displaystyle{x}={8}\usepackage{cmbright}\Huge\$ , $\displaystyle{y}={0}\usepackage{cmbright}\Huge\$ . When $\displaystyle{x}={4}\usepackage{cmbright}\Huge\$ , $\displaystyle{y}={2}\usepackage{cmbright}\Huge\$ . When $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ , $\displaystyle{y}={4}\usepackage{cmbright}\Huge\$ . Check your work by substituting both values in at once.