documentclass[11pt,a4paper]{article}
      usepackage{blindtext}
      usepackage{mathtools}
      DeclareMathOperator{sgn}{sgn}
begin{document}
begin{flalign*}
a^{m}times a^{n}&=
a^{m+n},\
intertext{and generally}
a^{m}times a^{n}times a^{p}times dotsm&=
a^{m+n+p+dotsb}
end{flalign*}
end{document}