In how many ways we can give 5 apples 4 mangoes 3 oranges ( fruit of same spacies ate similar) to three person if each may recieve none or more?

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.