a+b+c=0,-----(1)
4a+2b+c=3,-----(2)
a-b+c=6-----(3)
(2)-(1),3a+b=3----(4)
(1)=(3),2b=-6,b=-3.
3a-3=3,3a=6,a=2.
2-3+c=0,c=1.