Find the amount Enca owes at the end of 3 years if $6500 is loaned to her at a rate of 4% compounded monthly

Let x have probability generating function gx (s) and let un generating function u(s) of the sequence uo, u1, satisfies p(x > n). show that the (1- s)u(s) = 1 - gx(s), whenever the series defining these generating functions converge.