Finding the Equation of a Vertical Line or the Equation of a Horizontal Line
The lines having the simplest equations are the vertical and the horizontal lines.
Vertical Lines
The equation of any vertical line (whose slope is undefined) is 
where k is the x-coordinate of every point on the line.
Example 7. Find the equation of the line graphed above:
Answer:
Since the line is vertical and every point has x-coordinate -2, the equation is 
Example 8. Find the equation of the line through the points (2,-3) and (2,4).
Answer:
Since one of these points lies directly above the other, the line is vertical and thus has the equation 
Note that the slope cannot be computed for this line.
Example 9. Find the equation of the line whose slope is undefined and which passes through the point (6, 0).
Answer:
The line is vertical since its slope is undefined. Its equation is 