Creating an environment with an equation & split?

Question

Following this resource, I tried creating an environment like the following:

\newenvironment{splitEq}{
    \begin{equation}
    \begin{split}
}
{
    \end{split}
    \end{equation}
}

And using it like this:

\begin{enumerate}[label=(\alph*)]
    \item 
    \begin{splitEq} % Line 209
        a + b & = c \\ %example
        & = abc
    \end{splitEq}
\end{enumerate}

I get this error:

\begin{split} on input line 209 ended by \end{splitEq}.

It's my first time using LaTeX so I don't know if I'm misunderstanding something basic.

Thank you

Answer 1

As @DavidCarlisle said, you can't use the amsmath environments straightforwardly inside \newenvironment. But you can grab the entire environment body of your environment and use that to call the amsmath environments in a way they see each other. Using \NewDocumentEnvironment this is pretty easy:

\documentclass{article}

\usepackage{enumitem}
\usepackage{amsmath}

\NewDocumentEnvironment{splitEq}{b}
  {%
    \begin{equation}%
      \begin{split}%
        #1%
      \end{split}%
    \end{equation}%
  \ignorespacesafterend}
  {}

\begin{document}
\begin{enumerate}[label=(\alph*)]
    \item 
    \begin{splitEq} % Line 209
        a + b & = c \\ %example
        & = abc
    \end{splitEq}
\end{enumerate}
\end{document}