Now use the moment generating function from Exercise 5 to verify that, if X is Negative Binomial

Question:

Now use the moment generating function from Exercise 5 to verify that, if X is Negative Binomial with parameters p, r, then E(X2) = r(r + 1 - p)/p2.
In Exercise 5
P(X = x) = px(x) = (1- p)x-lp
For x = 1, 2, 3,...,
and pX(x) = 0 otherwise), then find the moment generating function of X.