When solving the equation such as 2/3 x = 4/5 , why would you multiply both sides of the equation by the reciprocal of ?

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.