Here is my question:
Consider a multiset $\{n\cdot a, n\cdot b, 1,2,3, \ldots, n+1\}$ of size $3n+1$. Determine the number of $n$-combinations.
I know from my textbook that if you have a $n$-combination multiset of $k$ types, then the answer is $\binom{ n-k-1} {k-1}$ but I am unsure what is $k$ and what is $n$ here. Any help is appreciated.