By the definition of big-Oh, we need to find a real constant $c > 0$ and an integer constant $n_0 \geq 1$ such that $2^{n+1} \leq c(2^n)$ for $n \geq n_0$. One possible solution is choosing $c = 2$ and $n_0 = 1$, since $2^{n+1}=2\cdot 2^n$.