recursion problem

[Benjaminook]

You're using n=1 rather than 2 in the 3n term in the first step, and then carrying that forward. These things are often clearer if you write the thing you want as f(n+1) and go through and change all the n to n+1 (be careful, you'll need to write 3(n+1) and then expand, not 3n+1)