How do I prove this combinatorics formula? $ 3^n = \sum_{k=0}^n\binom{n}{k}2^k $

Question

I am stuck on trying to prove this combinatorics question. The question reads:

Prove that for any positive integer n, $$ 3^n = \sum_{k=0}^n\binom{n}{k}2^k $$ Does anyone know how to begin/prove this? Thank you!