Your approach is correct.
One of the questions in my probability homework reads:
X denotes a negative binomial random variable, with p = 0.6 Find P(X ≥ 3) for a) r = 2 and b) r = 4.
According to my teacher, the answers are 0.1792 and 0.45568, respectively, but I can't for the life of me figure out how he got them. I tried finding P(X ≥ 3) by turning it into 1 - P(X ≤ 2) and then calculating p(2), p(1), and p(0), but I kept getting 0 for my answer, which obviously isn't correct.
Can someone please help me solve this problem, or explain to me how I would go about solving it? I'm really confused.
Edit: I also don't understand part b. I don't understand which numbers go where. Can you please help me?
The pmf is given here: Negative binomial distribution - Wikipedia, the free encyclopedia
and all you do is substitute the given numbers into it. Where do you get stuck in doing this?