Can someone answer these questions?

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