The given system is
[tex]\begin{cases}3x-y=6 \\ -3x+y=-6\end{cases}[/tex]We have to combine the equations. As you can observe, if we combine the like terms, we are going to get zero.
[tex]\begin{gathered} 3x-3x+y-y=6-6 \\ 0x+0y=0 \\ 0=0 \end{gathered}[/tex]