Question 92823
Recall that the slope (m) can be described as "Rise over Run" or the change in the y-coordinates over the change in the x-coordinates. In algebrese:
{{{m = ((y[2]-y[1]))/((x[2]-x[1]))}}}
Of course, the x's and y's are the coordinates of the given points (-4, 0) and (-4, -2)  So let's calculate the slope, m, of the line passing through the given points using the formula:
{{{m = ((-2-0))/((-4-(-4)))}}} Simplifying this we get:
{{{m = -2/0}}}...but, as you know, division by zero is frowned upon in mathematics, so when you see a slope that has zero in the denominator, (a zero run), this is called an "undefined (not a zero) slope". 
 What it really means is that the line is vertical.