The fastest way to understand a solution is to click on a particular step, which provides the logic.  It is also worth recommending that the 'Options' tab is used to control the number of solution steps.  My preference is to start with 'a few steps' and increase the number of solution steps only as the additional steps are helpful. 

The variable 'x' is being spit into "-7x" and "8x" as a step towards factoring.