\documentclass{article}
\usepackage{amsmath}
\usepackage{extarrows}
\begin{document}
% with amsmath
\[a\xrightarrow[r_2\times\frac12]
{\substack{r_1-\frac{\lambda}{2} r_2\\ r_3+\frac{\lambda}{2}r_2}} b\]
% without amsmath
\[a\xrightarrow[r_2\times\frac12]
{r_1-\frac{\lambda}{2} r_2\atop r_3+\frac{\lambda}{2}r_2} b\]
\end{document}