When the problem says "solve for y," the intent is simply to get y by itself.
You're on the right track, but:
5x + 6y = 18
5x - 5x + 6y = 18 - 5x (here, you added 5x to the right hand side)
6y = 18 - 5x
6y/6 = (18-5x)/6
y = 3 - 5x/6 (When you divide by 6, you have to divide EACH TERM by 6 - this means you need to do 18/6 AND 5x/6)
So, you were on the right track, just a couple small things.