From the above graph:
If you were to take 2 arbitrary points on the line and determine the slope, the following would be the result;
Taking coordinates (1,1) and (1,2), we get:
Since, anything divided by zero equals Undefined.
Therefore, the Slope(m) of the line is Undefined.
Even though it may seem irrelevant to the question, I will explain why the slop is undefined.
Here is one of the many reasons.
By our rule that if
Therefore, the line is undefined.