help

$\begin{pmatrix} n\\ a \end{pmatrix} = \frac{n!}{(a)!(n-a)!}$

substitute  n-1  for  n  and simplify.   Then you will have your answer.

Give it a go and get back with your answer or your difficulty.  

Please no one answer over the top of me, wait for guest to give a proper response.  Thanks.