From here it should just be a lot of algebra.
If you knew it to there then you should have posted that fact so that time and effort was not wasted showing you what you could already do.
I do not see how you can get the so-called answer of from all this. Unless the question asks for a specific form for the answer to be given in, the question has been answered in post #2.
There is no "equation" to "solve".
You were asked to compose the functions, and probably then to simplify the resulting expression. That's all.