Perpendicular lines have negative reciprocal slopes. That means the fraction is flipped and multiplied by negative 1. So if one line has a slope of 3/5, then a line perpendicular to it has a slope of -5/3.
A line perpendicular to another has a slope that is the negative reciprocal of the slope of the other line. The negative reciprocal of the original line is –2, and is thus the slope of its perpendicular line
