Answer:
The slope of this line is undefined.
Step-by-step explanation:
The slope of a line is rise over run, where the rise is the change in y value and the run is the change in x. Since there is no change in x when a line is parallel to the y-axis, any slope would mean you would divide by 0.
Since you cannot divide by 0 the answer will always be undefined no matter what the numerator is. The slope of any vertical line will be undefined.