You correctly found the value of x to be $\displaystyle{2}\usepackage{cmbright}\Huge\$ , but later made a mistake to get $\displaystyle{z}=-{3}\usepackage{cmbright}\Huge\$ instead of $\displaystyle{z}={3}\usepackage{cmbright}\Huge\$ . You substituted $\displaystyle{z}=-{3}\usepackage{cmbright}\Huge\$ and $\displaystyle{x}={2}\usepackage{cmbright}\Huge\$ in an original equation to get $\displaystyle{y}={10}\usepackage{cmbright}\Huge\$ . These values do not make the first and third equations true, however.