As you drag the points above notice that the slope indicator goes away when the line is exactly vertical. You may be able to guess that vertical lines are lines that go straight up and down but did you know that all vertical lines have the same slope. The one thing that is true about vertical lines is that the x value is constant.
If this doesnt make sense let me give you an example.
The slope of a vertical line is undefined because the denominator of the slope the change in X is zero. A ratio comparing the change in y the rise with the change in x the run is used calculate the slope of a line. So 100 again dividing by zero is undefined. Find the slope and the equation for a line through points -3 6 and -37.