Question 1011495
when two lines are parallel their slopes are the {{{same}}} 

since the line {{{x=4}}} is a vertical line, its slope is undefined

Is the vertical line going up on one end? Well, kind of. Is the vertical line going down on the other end? Well, kind of. Is there any number that is both positive and negative? Nope.

vertical lines have NO SLOPE. In particular, the concept of slope simply does not work for vertical lines. The slope doesn't exist! Let's do the calculations. 
I'll use the points ({{{4}}}, {{{5}}}) and ({{{4}}}, {{{-3}}}); the slope is:

{{{m=(5-(-3))/(4-4)=8/0=undefined}}}


the line that is perpendicular to the vertical  line is the horizontal line, for example {{{y=4}}}


Is the horizontal line going up; that is, is it an increasing line? No, so its slope won't be positive. Is the horizontal line going down; that is, is it a decreasing line? No, so its slope won't be negative. What number is neither positive nor negative? {{{Zero}}}! 
So the slope of this horizontal line is: 

{{{m=0}}}