See, pls, offered explanation:
1) the both equations are linear function, it means system made up of the both equation has the only solution.
2) if to prove this:
[tex] \left \{ {{-x-y=3} \atop {-x+y=-3}} \right. \left \{ {{x=0} \atop {y=-3}} \right. [/tex]
Answer: one solution.
