Question 647743
A  the slope of a horizontal line is {{{0}}}, and the slope of a {{{vertical}}} line is {{{undefined}}}. 

A vertical line has undefined slope because all points on the line have the same {{{x-coordinate}}}. 
As a result the formula used for slope {{{(y2-y1)/(x2-x1)}}} has a denominator of {{{0}}}, which makes the slope {{{undefined}}}.


plot a point (1,4) and draw a vertical line through it