9. In a system of equations both equations have to work for the same x And y.
If we substitute x being -1 in the given equations then the y in both equations better be the same.
-(-1)+2y=6 there fore y = 5/2 also
6y= -1+18 there fore y=17/6 not good because I got two different y for the same system of equations
Now we try x being 0
-0+2y=6 so y=6/2=3
6y= 0+18 so y=18/6=3 yeah for x=0 and y=3
Both equations of the system work.
Keep doing the same for the rest.