Consider the line y=5x-7
What is the slope of a line parallel to this line?
What is the slope of a line perpendicular to this line?
The slope of a line parallel to y=5x-7 is also 5, because parallel lines have the same slope.
The slope of a line perpendicular to y=5x-7 can be found by taking the negative reciprocal of the slope of the original line. The slope of the original line is 5, so the slope of a line perpendicular to it would be -1/5.