As far as I can see there is no simplification.. A fraction is zero only when the numerator is 0. They have just used that idea.
I am not sure what you refer to as simplification, tell us where you want to do it and how? Then we can comment on it.
For this question, rather than simplifying the R^2 and the R^3, they just move the R^2 over and times by 0, etc.
I'm wondering, why not simplify? They simplified to a degree before hand, so why not now? How do you know when to stop? They get the right answer of course, but how do I know when to simplify and when not to?