documentclass{beamer}
 usepackage[T1]{fontenc}
    %-----
    usepackage{amsmath,amssymb,amsfonts} %math
   usepackage{graphicx}
   usepackage{color}
     usetheme{Madrid}

      begin{document}

     begin{frame}{}
      begin{align*}
          |x_n-x_m|_B
          &=|B(x_n-x_m)|pause\
          &=|y_n-y_m|,
          end{align*}
       end{frame}

 end{document}