for tobi..

2x+y-4z=-2
x-3y-9z=6
x+ay+bz=2

2x+y-4z=-2 -> multiply with (-3)
-6x-3y+12z=6 also x-3y-9z=6
therefore,
-6x-3y+12z=x-3y-9z

x-3y-9z=-3(2x+y-4z)
x-3y-9z=-6x-3y+12z
7x=21z
x=3z

6z+y-4z=-2
2z=-2y-2
2z= -2(y+1)
-z=y+1
z=-y-1

x=3z
z=-y-1
x=-3y-3

2(-3y-3)+y-4(-y-1)=-2
-6y-6+y+4y+4=-2
y-2=-2
y=0, z=-1, x=-3

-3+a.0+b.(-1)=2
-3+0+b=2
b=-5
a= anything