You have the following polynomial:
2r - 4r + 9 + r
In order to simplify the previous polynomial you take into account that you can simplify ONLY terms that are similar. Similar terms are those with the same variable. Then, you obtain:
2r - 4r + 9 + r = (2r - 4r + r) + 9
= -r + 9
In the simplification you operate onl with the coefficents of the terms.
