Question 1097284
Multiplying 2x + y- z = -5 times -1 gives you -2x - y+ z = 5
If you add that and the other two equations, you get
-2x - y  +  z = 5
-5x - 3y + 2z = 7
  x + 4y - 3z = 0
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-6x + 0y + 0z = 12
That is
{{{-6x = 12}}} ---> {{{x=12/(-6)}}} ---> {{{highlight(x=-2)}}} .