You correctly found the value of z to be $\displaystyle-{4}\usepackage{cmbright}\Huge\$ , but later made a mistake to get $\displaystyle{y}=-{2}\usepackage{cmbright}\Huge\$ instead of $\displaystyle{y}={2}\usepackage{cmbright}\Huge\$ . You substituted $\displaystyle{z}=-{4}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}=-{2}\usepackage{cmbright}\Huge\$ in the first equation to get $\displaystyle{x}={9}\usepackage{cmbright}\Huge\$ . These values do not make the second and third equations true, however.