example showing the use of references for formulas. I made it compiling the following code:

\documentclass{article}

\usepackage[english]{babel}

\begin{document}

\begin{equation} \label{eq:solve}
x^2 - 5 x + 6 = 0
\end{equation}

\begin{equation}
x_1 = \frac{5 + \sqrt{25 - 4 \times 6}}{2} = 3
\end{equation}

\begin{equation}
x_2 = \frac{5 - \sqrt{25 - 4 \times 6}}{2} = 2
\end{equation}

and so we have solved equation \ref{eq:solve}

\end{document}