The slope of a line in graph or any system could be given as the ratio of the change in the y coordinates to change in the x coordinates
Here, it is given that the vertical line which passes through a single coordinate in the x-axis. There is only one coordinate is available in the x-axis. There are several points is available in the y coordinates.
Hence, it is defined as undefined because, if the denominator has zero then it becomes undefined.
Now, it is understood that the reason why the line has given has a slope undefined.
