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.