\documentclass{article}
\begin{document}

\begin{picture}(1,1)
\put(0,0){\line(1,0){100}}
\put(0,0){\line(0,1){100}}
\put(30,10){${\bf r}_2$}
\put(5,30){${\bf r}_1$}
\put(45,45){${\bf r}_{12}=|{\bf r}_1-{\bf r}_2|$}
\thicklines
\put(0,0){\line(-1,-1){50}}
\put(0,0){\vector(2,1){60}}
\put(0,0){\vector(1,2){30}}
\put(30,60){\line(1,-1){30}}
\end{picture}

\end{document}