If you do not understand a single step, even the first one, then either you have not worked hard enough at it or you do not have the prerequisite math to understand it. Every step is very routine algebra. Algebra is a prerequisite to understanding calculus.
When you attach a file such as you have done here and elsewhere, please click on "Full Image". As a Homework Helper here at PF, I find it very annoying to have to open a new window to view the full sized image.