The slope of the line between the points = (change in 'y') / (change in 'x')
Change in 'y' = (-3) minus (-13) = -10
Change in 'x' = (-4) minus (-4) = zero
Slope = (change in 'y') / (change in 'x') = (-10) / (0) .
Big problem ! Division by zero is undefined, and we're
not allowed to even think about doing it.
So we just have to take a couple of steps back and LOOK at it.
We have two points at the same 'x' ... -4 ... but different 'y' values.
If you can picture those points on a graph, you'll see that
the line between them is a VERTICAL line.
Its slope is 'undefined'. (We really want to call it 'infinity'.)
