Dear Sylvain,

I am using ’\startalign’

> 
> \starttext
> \startformula
> \eqalign{
> ax^2+bx+c &= 0 \cr
> x &= \frac{-b \pm \sqrt{b^2-4ac}}{2a} \cr
> }
> \stopformula
> \stoptext
> 

\starttext
\startformula
\startalign
\NC ax^2 + bx + c \NC = 0 \NR
\NC x \NC = = \frac{-b \pm \sqrt{b^2-4ac}}{2a} \NR
\stopalign
\stopformula
\stoptext

There is  a nice guide book "My Way” for math-alignment by Aditya.
https://wiki.contextgarden.net/images/archive/b/b4/20100530075041!Mathalign.pdf <https://wiki.contextgarden.net/images/archive/b/b4/20100530075041!Mathalign.pdf>

Best regards,

Dalyoung