Left equation numbering

By default, equation numbers are place on the right side of an equation for any numbered math environment, such as equation, eqnarray, and align. Surprisingly, the equation number position can is changed through the optional documentclass argument leqno:

Left numbering

\documentclass[leqno]{article}

Right numbering (by default and therefore usually skipped):

\documentclass[reqno]{article}

Here is a minimal working example:

\documentclass[a4paper,12pt, leqno]{article}
\usepackage{amsmath}
\begin{document}
\subsubsection*{Equation:}
$$f(x)=ax^2+bx+c$$
\subsubsection*{Align:}
\begin{align}
f_1(x)&=a_1x^2+b_1x+c_1\\
f_2(x)&=a_2x^2+b_2x+c_2
\end{align}
\end{document}

Align is part of the amsmath package which was used here because there are several reasons to avoid eqnarray.