Hi Guys,

Doing some practise papers for my DM final and have come across something I don't quite understand. I didn't do high school math (last time I did any was 8 years ago) so I find this courseverychallenging/frustrating but enjoyable!

Question

Write down:$\displaystyle \left(\begin{array}{cc}n\\n-3\end{array}\right)$

My answer:

$\displaystyle \frac{n!}{(n-3)!(n-(n-3))!}$

But the actual/final answer should be:

$\displaystyle \frac{n(n-1)(n-2)}{6}$

Can anyone explain how to get this? I understand factorials, but only the basics when variables are involved. Thankyou!