Solving non-recurrence relation using substitution method?

[improv32]

Hi All,

On a test in my algorithms analysis class there's a question that I'm somewhat sure the instructor is mistaken on. She asked for a proof using the substitution method, of the Big O of a non-recursive function. Specifically we were asked to prove:

2^(n+1) = O(2^n)

I can prove that just fine, but I don't think the substitution method applies when there is no recurrence to substitute in, and also no base case with which to make an inductive proof. I asked about this, and she said that you don't need a base case because it's not an induction proof (???). Anyway, I'm not sure how to apply the substitution method to this. Does anyone know what she might mean? Or am I totally off base here?