Keep in mind that you can gather only like terms. Two terms can be summed/subtracted if they involve the same powers of the same variables.
So, going point by point:
- We can subtract the terms, because they both involve the variable
, and we have
- We can sum the terms, because they all involve the variable
, and we have
- We can't sum this terms: 3 is a pure number, while 3r involves the variable r.
- We can only sum the two terms: we have
- We can't sum this terms: -20 is a pure number, while -15p involves the variable p.
- We can sum the terms, because they all involve the variable
, and we have