If this sort of algebra is an obstacle, you should do some practice from the relevant parts of the Foundations of Math book. Since that book doesn't contain a ton of problems, it would probably be in your interest to pick up a high-school algebra textbook (e.g., from a public library, or from an online seller of used books) and run through a bunch of practice problems.
You should try to get to a point where these types of calculations become more or less automatic.