From the 1st eqn X2=-X1, and we can substitute this value for X2 in eqn 2: -X1+X3=-8.
Add this to eqn 3: 2X3=-4, so X3=-2.
Using eqn 3 again we have X1-2=4, so X1=6 and therefore X2=-6.
Test all these values in the three eqns to prove that all are satisfied.