He said the man was historical. Written homework problems should be self-explanatory. Slope is change in y divided by change in x . Note very carefully that if a line has a slope of 0, it has a slope. This is different than saying the slope is undefined.
Parallel lines have equal slopes.
You should realize that a horizontal line is perpendicular to a vertical line. Now, consider two lines, neither of which is horizontal or vertical. They will be perpendicular is the product of their slopes is -1. Math is a language . Evanston, IL, McDougal Littell , 2001.