\documentclass{article}
\usepackage{amsmath}
\usepackage{cases}
\usepackage{bm}
\begin{document}
\begin{numcases}{a=}
\dot{\bm{\lambda}} (t) &$= -\frac{\partial H}{\partial \bm{x}}$, \label{2-2-4} \\
\frac{\partial H}{\partial \bm{u}} &$= 0$, \label{2-2-5} \\
\dot{\bm{x}} &$= \bm{f} (t,\bm{x}, \bm{u})$, \notag
\end{numcases}
\end{document}
related link
\documentclass[12pt]{ctexart}
\usepackage{amsmath}
\usepackage{empheq}
\usepackage{bm}
\begin{document}
\begin{equation}
a^2+b^2=c^2
\end{equation}
\begin{empheq}[left={a=\empheqlbrace}]{align}
\dot{\bm{\lambda}} (t) &= -\frac{\partial H}{\partial \bm{x}}, \label{AA} \\
\frac{\partial H}{\partial \bm{u}} &= 0, \label{BB} \\
\dot{\bm{x}} &= \bm{f} (t,\bm{x}, \bm{u}), \notag
\end{empheq}
\begin{equation}
a^2+b^2=c^2
\end{equation}
\end{document}
多谢大佬回复
@u40 这里这个奇怪的空格是怎么出现的捏

@u70550 看了下文档,本来就是这样的,不过这个空格是用于分割
<explanation>
用的,用于=
似乎不是完全合适。@u70550 微调