Find the slope of the line through each pair of points (-4,-13), (-4,-3)

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'.)