\documentclass[a4j,12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{eqnarray}
 \dot{\varepsilon}_{xx} = - \dot{\varepsilon}_{yy} 
  = \frac{12V_a D_0\sigma}{RTh^2}
  \exp\left(
       -\frac{E_a +pV_a}{RT}
     \right)
\end{eqnarray}
\end{document}
