Parameters for negbinomial Function?

[mark4flies]

I do not understand the description of the negbinomial function in the Help system. My understanding of this probability mass function is that it provides the probability of r successes out of x trials when the probability of a success is equal to p. The HP Prime computation is P(X=x) = COMB(x-1,r-1)*(p^r)*(1-p)^(x-r). I defined a user function this way and confirmed its results.

I want to use the built-in function negbinomial(n,k,x) but I do not understand the parameters. How do n, k, and x here relate to the x, r, and p above?

Thanks!