Big Oh rules - How to argue in case of negative base
If $C_n = -2^n + 2^n$, then this just simplifies to $C_n = 0$, so I assume that you mean $C_n = (-2)^n + 2^n$. Then:
$$
C_n \le |-2|^n + 2^n = 2^{n+1} = O(2^n).
$$