Slope of a line, when given two points

What is the slope of the line that passes through the points (2, -1) and (2,0)?

(2,-1) , (2,0)

y2 - y1
-------- = slope  {change in y coordinates over the change in x coordinates}
x2 - x1

 0 - (-1)
---------  {substituted into slope formula}
  2 - 2

1
---  {subtracted}
0

= undefined  {it is impossible to divide a number by 0}

The slope is "undefined". Also, this would be a vertical line.
Horizontal lines have slopes of 0, such as 0/3.

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