Adding right brace and equation number

Question

I have an eqnarray that consists of 4 lines. I would like to have a right brace } that spans the last two lines with equation number, first equation with equation number and second equation with nonumber. Some brief text explaining these two parts of the equation. Something like

a=b   (1)
c=d
x=y}
   }  (2) 
z=t}

Answer 1

Here is a quick solution. It uses a manually set size of the brace. Maybe someone can provide something more robust.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
a &{} = b \\
c &{} = d \nonumber \\
\begin{split}
    x &{} = y \\
    z &{} = t
\end{split} \quad \bigg \rbrace \quad \text{comment text}
\end{align}

\end{document}