Here I need the lateral surface area

Evaluate the expression for the given value of the variable 7a - 4a for a =8

In the next 10 month, colin wants to save $900 for his vacation. he plans to save $75 each of the first 8 months. how much must he save each of the last 2 months in order to meet his goal if he saves the same amount each month ?

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.