how can i prove this equation of binomial coefficients?

Question

i dont know $-a$ in this equation really mean, explain it too please $$ \left(\begin{array}{c}a+m-1\\ m\end{array}\right) = (-1)^m \left(\begin{array}{c}-a\\ m\end{array}\right) $$

Answer 1

A generalized binomial coefficient $\binom{x}{m}$ with $m\in\mathbb N$ but $x$ not necessarily a natural number is given by $$\frac{x(x-1)\cdots (x-m+1)}{m!}$$ Can you use this to prove it?