Question 401621
Any two points on a horizontal line have coordinates (a, b) and (c, b).  The slope is then equal to {{{(b-b)/(c-a) = 0/(c-a) = 0}}}, given a is not equal to b.

Any two points on a vertical line have coordinates (d, e) and (d, f) (assuming e not equal to f).  The slope is then equal to {{{(f-e)/(d-d) = (f-e)/0}}} = undefined, because division by 0 is impossible.