Set the two equations equal to eachother, and solve for y
The answer I have highlighted answers the question. So this answer is fine. Stop now - no need to worry any further.
I never could see the point in further re-working of a perfectly good answer. Every further bit of unnecessary additional working creates the opportunity for making a mistake. If your final answer is wrong, you will lose the answer mark, regardles of whether or not an earlier answer was correct. And as it happens, your last line is wrong. Which only goes to prove my point.