Question

How do you find the slope of the line `x=1`?

Answer 1

Slope is undefined that is `oo`
Reason:
The line `x = 1` is an ordinate and all ordinates are parallel to the y-axis.

Another way of thinking of this is that it has an infinitely large slope.

Answer 2

The definition of slope requires 2 points on a non-vertical line. Alternatively, it requires two points `(x_1, y_1)` and `(x_2, y_2)` with `x_1 != x_2`.

The slope of the line `x=1` does not exist. Because there are no tow points with different `x` values on the line.
It is not defined.
This case is not includes in the definition of slope.

(There is also no-slope intercept form, because there isn't a slope to use for `m`.)