Solution
\begin{split}
y &= f(x) \\
&\qqud g(x)
\end{split}
Reference
https://tex.stackexchange.com/questions/358608/split-equations-inside-the-align-environment
\begin{split}
y &= f(x) \\
&\qqud g(x)
\end{split}
https://tex.stackexchange.com/questions/358608/split-equations-inside-the-align-environment