All, dvi attached.

\documentclass[leqno]{amsldoc}
\usepackage{amsmath}
\begin{document}

You can put the tags on the left with the legno option.

You can align equations with automatic numbering each line.
\begin{align}
2x+3&=5\\
2x+3-3&=5-3\\
2x&=2\\
\frac{2x}2&=\frac22\\
x&=1
\end{align}

You can suppress numbering.
\begin{align*}
2x+3&=5\\
2x+3-3&=5-3\\
2x&=2\\
\frac{2x}2&=\frac22\\
x&=1
\end{align*}

You can have subnumbering.

\begin{subequations}
\begin{align}
2x+3&=5\\
2x+3-3&=5-3\\
2x&=2\\
\frac{2x}2&=\frac22\\
x&=1
\end{align}
\end{subequations}

You can interject text.

\begin{align*}
2x+3&=5\\
2x+3-3&=5-3  &  \text{Subract 3 from both sides}\\
2x&=2  &  \text{Simplify}\\
\frac{2x}2&=\frac22  &  \text{Divide both sides by 2}\\
x&=1 & \text{Simplify}
\end{align*}

You can align more than one set of equations.

\begin{align*}
2x+3&=5                 &  r&=0\\
2x+3-3&=5-3             & 1+2\sin\theta&=0\\
2x&=2                   & \sin\theta&=-\frac12\\
\frac{2x}2&=\frac22     & \theta&=\frac{7\pi}6\\
x&=1
\end{align*}

\end{document}