Can you explain in your own words why the line x = 4 is a vertical line.

Answer by  wxwnnr (13)

In the (x-y) plane, x=4 is not a function of y, so y can take on any value. For each of these values x is always 4.

Answer by  Chrissie (266)

For any point on your graph where x=4, such as (4,0) or (4,16) or (4,-5), all these points will be in a straight line. Plot them and see. And since you can always find a point in between two others, like (4,5. 1) or (4,5. 0001), points infinitely close together make a line.