How to left-align cases inside an equation?

Question

I am trying to left-align a cases inside an equation, like this:

and I need it to be inside equation because of the number (16). When I try this:

\begin{equation} \label{eq:evaluate_metrics}
\begin{align*}
    &\begin{cases}
        TPR=T_P/(T_P+F_N)\\
        FPR=F_P/(F_P+T_N)\\
        ACC=(T_P+T_N)/(T_P+F_N+F_P+T_N)\\
        PRE=T_P/(T_P+F_P)
    \end{cases}
\end{align*}
\end{equation}

gives me an error:

Package amsmath Error: Erroneous nesting of equation structures;(amsmath) trying to recover with `aligned'. \end{align*}

How can I make it look like the picture while including the equation number?

Answer 1

You can obtain locally left-aligned equations nesting them inside a fleqn environment from nccmath. This environment can take the distance from the text left margin at which it starts as an optional argument:

\documentclass{article}
\usepackage{amsmath, nccmath}
 \usepackage[upright]{fourier} 
\usepackage[reset, showframe]{geometry}

\begin{document}

\begin{fleqn}
\begin{equation} \label{eq:evaluate_metrics}
    \begin{cases}
        TPR=T_P/(T_P+F_N)\\
        FPR=F_P/(F_P+T_N)\\
        ACC=(T_P+T_N)/(T_P+F_N+F_P+T_N)\\
        PRE=T_P/(T_P+F_P)
    \end{cases}
\end{equation}
\end{fleqn}

\begin{fleqn}[1em]
\begin{equation} \label{eq:evaluate_metrics}
    \begin{cases}
        TPR=T_P/(T_P+F_N)\\
        FPR=F_P/(F_P+T_N)\\
        ACC=(T_P+T_N)/(T_P+F_N+F_P+T_N)\\
        PRE=T_P/(T_P+F_P)
    \end{cases}
\end{equation}
\end{fleqn}

\end{document}