Equation sub-numbering

Question

How can I label my equations as:

a = b+c (1(a))
d = f+g (1(b))

Please advice as I have used simple labeling structure \label but don't know how to handle the letters.

Answer 1

You can use the subequations environment from amsmath:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{subequations}
\begin{align}
y & = d \\
y & = cx + d \\
y & = bx ^{2}+ cx + d \\
y & = ax ^{3}+ bx ^{2}+ cx + d
\end{align}
\end{subequations}

\end{document}