More of same here, but look at the text at the bottom of this page. Aha! A definition! In passing, where most students would miss it, the concept of “like terms” is defined. Now we're getting somewhere, right?

Well, no. The example used here is with like terms that are simple integers. There is only one exercise in this chapter that requires the student to combine like terms that contain the variable as a factor in this chapter. The book does not explain why combining like terms is an application of the distributive law. It's a trick.