sequences and series - How many terms are 'missing'?

I know in this particular indeterminate partial sum S = $3^n - 3^{n+1} + 3^{n+2} - 3^{n+3} + \cdots + 3^{3n}$ where $a=3^n$ and $r=-3$. So I know if $3^1$ were the first term, there would be $3n$ terms. But I am missing $3^1,3^2,3^3, \cdots , 3^{n-1}$ (namely $n-1$ terms). Therefore $3n-(n-1) = 2n+1$ terms.

Now here's what I am having trouble with trying to apply the same process and find how many terms there are missing of this

$3^k + 3^{k-1} + 3^{k-2} + \cdots + 3^{-2k}$

it's hard to me now because it's counting down by 1. I believe that if $3^1$ was the first term there would be $-2k$ terms. But how do you find how many terms come before that? I was thinking like $3^{k+1 -1}$ comes before $3^k$ but am having trouble expanding the expression to make it clear to me what's happening to find how many terms are missing to get the number of terms there are. Also don't exactly know what the common ratio is but I see that the sign doesn't alternate so must have a positive value of r.

Basically, I want to apply the process used in the $3^n$ expression to that of the $3^k$ expression. Please help