As the line is vertical, it has same points for x - coordinates. And it never changes for whole line.
We know, slope (m) = y2 - y1 / x2 - x1
As x2-x1 = 0 [ x2=x1 ]
m = y2-y1 / 0
m = undefined
So, y = mx + c
y = c
In short, Your Answer would be y = c
Hope this helps!
