The presenter of the slide show perpendicular lines, which is located in the media section of chapter 3, states that the slope of the line l, that is perpendicular to the original line is (points : 1)

Perpendicular lines have negative reciprocal slopes.
so..the slope of the line l, that is perpendicular to the original line has a negative reciprocal slope.

Example : 
original equation ,slope = 2
perpendicular line, slope = -1/2

original equation, slope = -1/3
perpendicular line, slope = 3