Since your solving for y; then you got to isolate it, so bring x to the other side [ don't forget to switch the sign to negative if it was positive or vice versa]
-3y=6-x
Now to fully isolate y divide both sides of the equation by -3 since its the only thing that's still connected to y
-3y/-3 = 6-x/-3
y = 6/3 - x/3
or y = 2 - x/3