Number one Eq. with (number+a,b) in an Eq. array?

Question

I want to number two equations on one line of an equation array in the form of (number a, b), here the "number" should be the line number. Please see the following code, I try to achieve it with \eqno, but Latex keeps showing me

You can't use \eqno in math mode.

\begin{eqnarray}
\label{eq1} & a+b+c=0,\\
\label{eq2} & d+e+f=0,\\
\label{eq3} & m+n = 0, \quad s+t=0, \eqno{(\theequation{\text{a},\text{b}})}
\end{eqnarray}

Answer 1

Very likely I do not understand the question, you can try

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
\label{eq1}  a+b+c&=0,\\
\label{eq2}  d+e+f&=0,\\
\label{eq3}  m+n &= 0, \quad s+t=0, \tag*{(\theequation{\text{a},\text{b}})}
\end{align}
\end{document}

and let me know if this is what you want. I have a hard time understanding what that could be good for.

Perhaps you are looking for something like this.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
\label{eq1}  a+b+c&=0,\\
\label{eq2}  d+e+f&=0,\\
\label{eq3}  m+n &= 0, \quad s+t=0,\stepcounter{equation} \tag*{(\theequation{\text{a},\text{b}})}
\end{align}
\begin{align}
\label{eeq1}  a+b+c&=0,\\
\label{eeq2}  d+e+f&=0,\\
\label{eeq3}  m+n &= 0, \quad s+t=0,\stepcounter{equation} \tag*{(\theequation{\text{a},\text{b}})}
\end{align}
\end{document}