I made a recurrence tree and guessed that solution to $T(n)=2T(n-2)+n$ is $O(2^{n/2})$ and I am now trying to prove this through substitution. These are my steps so far, but I can't get it to pass for some reason:
\begin{align} T(n-2) &\leq c2^{(n-2)/2}\\ T(n) &= 2(c2^{(n-2)/2})+n\\ &\leq c2^{n/2} \quad (?) \end{align}
But as far as I see, it can never add up, because it will always be greater than the $+n$ term and times $2$. Any help will be much appreciated.