At some point when multiplying by $\displaystyle-{1}\usepackage{cmbright}\Huge\$ , you made a mistake with signs and found $\displaystyle{z}=-{5}\usepackage{cmbright}\Huge\$ instead of $\displaystyle{z}={5}\usepackage{cmbright}\Huge\$ . You used the incorrect value to find the values of x and y in the second and third equations. However, the values do not make the first equation true, so they are not a solution.