After eliminating the variables y and z, you made a mistake while solving for x in the equation $\displaystyle-{3}{x}={3}\usepackage{cmbright}\Huge\$ . You found $\displaystyle{x}={1}\usepackage{cmbright}\Huge\$ instead of $\displaystyle{x}=-{1}\usepackage{cmbright}\Huge\$ , and used this value to find the values of y and z. To be a solution, the values must make each of the equations in the system true. These values do not make the third equation true.