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: