Answer:
a) we know that this is convergent.
b) we know that this might not converge.
Step-by-step explanation:
Given the [tex]\sum^\infty_{n=0}C_n8^n[/tex] is convergent
Therefore,
(a)  [tex]\sum^\infty_{n=0}C_n(-3)^n[/tex] The power series [tex]\sum C_nx^n[/tex] has radius of convergence at least as big as 8. So we definitely know it converges for all x satisfying -8<x≤8. In particular for x = -3
∴ [tex]\sum^\infty_{n=0}C_n(-3)^n[/tex]  is convergent.
(b) [tex]\sum^\infty_{n=0}C_n(-8)^n[/tex] -8 could be right on the edge of the interval of convergence, and so might not converge