Question 714993
The line {{{x=-15}}} is what we call a "vertical" line, with "undefined" slope.
(It is vertical on my computer screen).
A line perpendicular to a vertical line is what we call a "horizontal" line, with {{{slope=0}}}.
See both lines in the drawing below.
 
{{{drawing(300,300,-27,3,-3,27,
grid(0),
blue(line(-15,-5,-15,30)),
green(line(-30,15,5,15)),rectangle(-15,15,-14.5,15.5),
circle(-8,15,0.2),locate(-12,14.5,P(-8,15)),
locate(-14.5,8,blue(x=-15)),
locate(-26,16.8,green(y=15))
)}}} The slope intercept form is {{{highlight(y=0*x+15)}}} or {{{highlight(y=15)}}} .
 
Pairs of perpendicular lines can present two different cases:
EITHER one line is vertical and the other is horizontal,
OR both lines have a slope and the product of the slopes is {{{(-1)}}}, as for perpendicular lines with slopes {{{3}}} and {{{(-1/3)}}}, or perpendicular lines with slopes {{{(-2/5)}}} and {{{5/2}}} .