\documentstyle[12pt]{article} 
\pagestyle{empty}
\begin{document}
\large
\begin{displaymath} 
\sf 
\frac{d \theta(\vec{r}_i,t)}{dt} =
\sum_{\vec{r}_j} w(\vec{r}_j,\vec{r}_i)\theta(\vec{r}_j,t)
- \sum_{\vec{r}_j} w(\vec{r}_i,\vec{r}_j)\theta(\vec{r}_i,t)
\end{displaymath}\
\end{document}
