$$x+2y-z=6 \times x-y-2z=12$$
$x+2y-z=6x-y-2z\text{ and }6x-y-2z=12$
$x=\frac{5z+36}{13}$
$y=\frac{4z+60}{13}$
$z=\frac{13y}{4}-15$
$x=\frac{5y}{4}-3$
