答案1
\documentclass{article}
\usepackage{amsmath}
\usepackage{enumerate}
\begin{document}
\begin{enumerate}[(a)]
\item
\begin{align}
y \equiv 0 &
\end{align}
\item
\begin{align}
\begin{cases}
y = \left(\frac{2}{3}x\right)^{3/2} & x \ge 0 \\
y = 0 & x \le 0
\end{cases}
\end{align}
\end{enumerate}
\end{document}