rather than the usual y=mx+c, a vertical line has an equation of x="x intercept". The "x intercept" is the co-ordinate where the line intercepts or cuts through the x axis.

Definition: A straight line on the coordinate plane where all points on the line have the same x-coordinate.
Draw a line between points A and B on the ground level.This is a vertical line.

The equation for a vertical line is X = a coordinate or location on the axis. This results in a vertical line at that point on the axis for all values Y.