Negative Binomial Distribution Problem

[mooshupork34]

I'm not sure how to go about this problem. Any help would be very much appreciated!:

Find the generating function for the negative binomial distribution below:

P(x=k) = (k-1 choose r-1) p^r * (1-p)^(k-r)

 

[pka]

Try this:
http://mathworld.wolfram.com/NegativeBinomialDistribution.html