What is a slope of a line perpendicular to the y-axis?

Such a line would have a slope of 0. An example is the line y=2. This line is perpendicular to X=0 (The y axis) . Slope is the change of y divided by the change in x of two points. There is no change in y so you get 0 divided by a constant, which is equal to 0.

0.

Slope is "rise over run", or (change in y) divided by (change in x).

A slope perpendicular to the y-axis is horizontal, and therefore (change in y) is 0. Since the denominator of the fraction is 0, the slope itself will be 0.

A line perpendicular to the y-axis is a horizontal line, so

Slope = 0

m = 0

The slope of the y-axis is undefined, but is perpendicular to the x-axis.

The x-axis has a slope m = 0 as does any line parallel to it.

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Zero, because the slope tells you how steep the line is. If it's horizontal, which it is, then it's not steep at all, therefore the slope is 0.