How do you write an equation of the line in standard form with vertical line through (21,-37)?

$\textcolor{g r e e n}{x = 21}$
For a vertical line, the value of $x$ is a constant.
If you insist on the explicit standard form: $A x + B y = C$
$\textcolor{w h i t e}{\text{XXX}} 1 x + 0 y = 21$