\nonstopmode
\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\pagestyle{empty}
\begin{document}
$$
{\tfrac  {1}{6^{3}}}(x+x^{2}+x^{3}+x^{4}+x^{5}+x^{6})^{3}
$$
\end{document}
