You made a mistake when solving for x and incorrectly found the value of x to be $\displaystyle-{3}\usepackage{cmbright}\Huge\$ instead of $\displaystyle{x}={3}\usepackage{cmbright}\Huge\$ . You used this value to solve for z in a previous equation to get $\displaystyle{z}=-{2}\usepackage{cmbright}\Huge\$ . You substituted $\displaystyle{x}=-{3}\usepackage{cmbright}\Huge\$ and $\displaystyle{z}=-{2}\usepackage{cmbright}\Huge\$ in the first equation to get $\displaystyle{y}=-{1}\usepackage{cmbright}\Huge\$ . These values do not make the third equation true, however.