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?

Bernard · Accepted Answer

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}

How to left-align cases inside an equation?

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