Explanation:
since both points have the save x coordinate, the line through both points is a vertical line through x = 4.
the slope of a line is the ratio
y coordinate difference / x coordinate difference
when b going from one point on the line to another.
since all points on the line have the same x coordinate, the x coordinate difference between 2 points is always 0.
and therefore, the slope is always
y coordinate difference / 0
which is a division by 0.
that means the slope is undefined.