Calculus question: power series

Suppose that SUM(0 to infinity)c(n)*x^n converges when x=-4 and diverges when x=6. What can be said about the convergence or divergence of the following:
a) SUM(0 to infinity)c(n)
b)SUM(0 to infinity)c(n)*8^n
c)SUM(0 to infinity)c(n)*(-3)^n
d)SUM(0 to infinity)(-1)^n*c(n)*9^n

I really have no idea where to even start with this one... Thanks in advance!

Note that the radius of convergence is between 4 and 6 (including 4).

a) |x| = 1 < 4; so this converges.

b) |x| = 8 > 4; so this diverges.

c) |x| = 3 < 4; so this converges.

d) |x| = 9 > 4; so this diverges.

I hope this helps!