Begin by going back and reviewing the basic algebra that this question depends on. Attempt easier questions as a lead-in to this question. If you do not understand the basic things there is no point trying to understand how they are applied to questions like this one. Your real problem is not this question, it is the underpinning algebra.
By the way, a solution may not seem to be explained very well if you are given guidance on how to solve the question but you do not understand the basic algebra underpinning that guidance.
What we don't like is people who insist on being given a complete solution that shows every single step.
Despite what you might think, it is not helpful to give you an entire solution. Post #2 gave you everything except the last step but you did not understand it. I have said the reasons why.
Do yourself a favor and review the basic material so that you can understand the help that people are trying to give you.
Thread closed.