teiothw.tex

\documentclass{article}

\usepackage{mathtools}
\DeclareMathOperator{\Barr}{Bar}
\newcommand\Fooo{\mathrm{Foo}}
\newcommand\Bazz{\mathrm{Baz}}
\newcommand\Fobb{\mathrm{Fob}}

\begin{document}
\begin{align*}
  \Fooo & = \Barr(x,y) \\ \shortintertext{where}
  x     & = \Bazz      \\
  y     & = \Fobb
\end{align*}
\end{document}