\documentclass[12pt]{article}

\begin{document}
  \thispagestyle{empty}

\begin{eqnarray*}
  E = 4\epsilon \left[ \left(\frac{\sigma}{r}\right)^{12}-\left(\frac{\sigma}{r}\right)^{6} \right]
% \qquad r < r_c \\
\end{eqnarray*}

\end{document}
