Compute following:
$$
\sum\_m\binom{n}{m}\sum\_k\frac{\binom{a+bk}{m}\binom{k-n-1}{k}}{a+bk+1}
$$
Only consider real numbers a, b such that the denominators are never 0.

Now I simplify it into
$$
-\frac{1}{n}\sum\_k\binom{n}{k}\binom{-n}{a+bk+1}(-1)^{a+bk+k}
$$
I have problem with this question in which I can't eliminate coefficient b.

But I can't find any formula to get answer.Please help me!