This is what the AMS math
\tag command is for.
x between parentheses, and in text mode.
$k$ above to get a label looking like a math-mode letter.
Use the star-form,
\tag* if you don't want the parentheses.
But with a narrow label, you may need some adjustments to get something neatly centred horizontally and vertically. Here I have hidden this complexity in a new