You can use \binom and \frac from the amsmath package to write the binomial coefficient expression.
Here is an example:
\documentclass{article}\usepackage{amsmath}\begin{document}\[ \binom{n}{k} = \frac{n!}{k!(n-k)!}\]\end{document}
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\binom{n}{k} = \frac{n!}{k!(n-k)!}
\]
\end{document}
The above latex code will display the following expression:
