Hi, I am encountering this problem:

Let a0, a1, a2.... be an infinite sequence of non-negative integers and A(x) = the summation of an * x^n is the corresponding generating function.

Express the following sequences in terms of A(x), and it should not contain a summation symbol or an infinite sum:

c) a0, 0, a2, 0 , a4, 0, a6.........

d) a0, 2a1, 4a2, 8a3, 16a4........

I have thought so hard but still nothing come out form my mind. thanks in advance.