Explain what the variable represents in an addition or subtraction expression

Me on this i beg you on this to answer!

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.

He factorization of 8x3 – 125 is (2x – 5)(jx2 + kx + 25). what are the values of j and k?