[Tex/LaTex] How to automatically number an equation like this

Question

How to automatically number an equation like this:

Answer 1

Not fundamentally different from GuM's answer, but some improvements to ease typing partial derivatives with esdiff and cross-referencing with cleveref (to be loaded after hyperref, if you use it):

\documentclass{book}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}
\usepackage{esdiff}
\usepackage{cleveref}
\setcounter{chapter}{2}

\begin{document}

To find $g$, we know that
\begin{subequations}
  \begin{align}
    \diffp{g}{x} & = 3x^2y^2 + x^2, \label{eq:1} \\%
    \diffp{g}{y} & = 2x^3y + y^2.\label{eq:2}
  \end{align}
\end{subequations}
Integrating \Cref{eq:1} with respect to $x$ gives us
\begin{equation}\label{eq:3}
  g = x^3y^2 + \frac{x^3}{3} + h(x)
\end{equation}

\end{document}