\documentclass[11pt]{book}

\begin{document}

\chapter{Introduction}

\section{one}
Here are equations~\ref{eqn:one} and \ref{eqn:two}:
\begin{equation}
\label{eqn:one}y = 3x+2
\end{equation}
\begin{equation}
\label{eqn:two}y = 5x-2
\end{equation}

\chapter{Discussion}
\section{two}
Here are equations~\ref{eqn:three} and \ref{eqn:four}:
\subsection{two point one}
\begin{equation}
\label{eqn:three}y = 3x+2
\end{equation}
\begin{equation}
\label{eqn:four}y = 5x-2
\end{equation}

\end{document}
