Manipulating PGF for Probability Calculation

[jackbauer]

Hi people, I was wondering if anyone could give me a hint with this problem.
A RV X has PGF Gx(s)= (4-s)/3(4-3s)

I need to get the P(X=i) from here. I know i can manipulate the above to get the sum to infinity of a geometric series, but i end up with the term
(4-s) still in the P(X=i) which i don't think can be right. Anybody got any hints on how to rearrange the above so as to eliminate the s in the numerator so I can obtain P(X=i)? Thanks a lot, Jack

 

[matt grime]

multiply out the (4-s) and the infinite series.