I have the following which works, except for the fact that the equation
number does not go to the right margin.

\startplaceformula
\startformula[align=middle, prefix=no]
\startcases[distance=2pt]
\NC \[21-(9 - n^2)\NC =□^\prime\]\NR
\NC\hfill n^2+12 \NC=□^\prime.\NR[eq04]
\stopcases
\stopformula
\stopplaceformula

I expect that I have missed something pretty basic here, so any guidance
will be greatly appreciated.

All best,
Alan