Labelling equations

Strange, I have opened (on a different installation) a document for which I
have never had this problem before, and don't know why it has suddenly
emerged. My equations are labelled numerically on the left, where previously
they were labelled on the right. Is there any way to change this back? I
include the following TeX to illustrate; I have put in some extra parameters
hoping that someone can also tell me, from what I have provided, how to
increase the space between the bottom line of the main text and the page
number.

\documentclass{amsart}
\usepackage[width=5in,height=7.7in,centering]{geometry}
\usepackage{amsmath,amssymb}
\usepackage{chngpage}
\linespread{1.083}
\setlength\headheight{\baselineskip}
\setlength{\parskip}{6pt}
\setlength{\headsep}{25pt}
\usepackage{fancyhdr}
\renewcommand{\footnoterule}{}
\renewcommand{\headrule}{}
\pagestyle{fancy}
\cfoot{\thepage}

\begin{document}


\begin{equation}
\label{eq:5}
a= b.
\end{equation}


\end{document}.

Regards,
Mark