This Instructional Activity is a part of the Lesson 15, Unit 4, Grade 8. When we began, we said that we wanted to define slope in a way so that a horizontal line would have a slope of because a horizontal line has no steepness or slant. What did you notice in Exercise 6? The slope of the horizontal line was zero, like we wanted it to be. In Exercise 3, you were asked to compare the steepness of the graphs of two lines and then compare their slopes. What did you notice?