Hello, kalagota

Use \label{} and \ref{} :

Code:

\begin{equation}
x=y
\label{name-of-this-equation}
\end{equation}
From equation \ref{name-of-this-equation} we have...

\ref{name-of-this-equation} prints the number of the equation labelled "name-of-this-equation". In case you don't know it : you must compile twice your document when you use \label{} and \ref{}.

Another possibility is to use the command \eqref{} (from the package amsmath) instead of \ref{}. \eqref{} puts labels of equations between parenthesis, \ref{} doesn't.