Prove: $T(n) = T(n-2)+2n$ is $O(n^2)$. 

I'm trying to solve by substitution method, what I reached so far is: 

$T(n-2)+2n$ = $T(n-4)+4n - 4$ = $T(n-6)+6n - 12$ = $T(n-8)+ 8n - 24$

How do I generalize this?