If the longest constraint needs to be flushed to the right margin, you could use a tabular*
environment with two centered columns; something along these lines (change the lengths according to your needs):
\documentclass[reqno]{amsart}
\usepackage{array}
\newlength\MyIndent
\setlength\MyIndent{10em}
\begin{document}
\vspace{\abovedisplayskip}
\noindent\begin{tabular*}{\textwidth}{@{\hspace{\MyIndent}}>{$}c<{$}@{\extracolsep{\fill}}
>{$}c<{$}@{}}
a^2 + b^2 = ab & (a \in \{z,y,x,w,v,u\},\ b \neq 0) \\
a + b = c & (a \in \{z,y\},\ b = 0) \\
\end{tabular*}\par
\vspace{\belowdisplayskip}
\end{document}
If the longest constraint doesn't need to be flushed to the right margin, a simple array environment will do the job (change the lengths according to your needs):
\documentclass[reqno]{amsart}
\begin{document}
\[
\begin{array}{@{\hspace{7em}}c@{\hspace{3em}}c}
a^2 + b^2 = ab & (a \in \{z,y,x,w,v,u\},\ b \neq 0) \\
a + b = c & (a \in \{z,y\},\ b = 0) \\
\end{array}
\]
\end{document}
I'd define a new environment, to which you give the desired label as argument.
\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newenvironment{varsubequations}[1]
{%
\addtocounter{equation}{-1}%
\begin{subequations}
\renewcommand{\theparentequation}{#1}%
\def\@currentlabel{#1}%
}
{%
\end{subequations}\ignorespacesafterend
}
\makeatother
\begin{document}
An equation
\begin{equation}
1=1
\end{equation}
Some subequations
\begin{varsubequations}{P1}
\label{eq:Parent}
\begin{align}
a& \label{eq:1}\\
b& \label{eq:2}
\end{align}
\end{varsubequations}
Another equation
\begin{equation}
2=2
\end{equation}
Now the references: \eqref{eq:Parent}, \eqref{eq:1}, \eqref{eq:2}.
\end{document}
Best Answer
if i correctly understood you, than you looking for this: