This Instructional Activity is a part of the Lesson 11, Unit 3, Grade 8. In this activity, they write equations for horizontal lines (lines of slope 0) and vertical lines and they graph horizontal and vertical lines from equations. Students explain their reasoning. Horizontal lines can be thought of as being described by equations of the form y = mx + b where m = 0. In other words, a horizontal line can be thought of as a line with slope 0. Vertical lines, on the other hand, cannot be described by an equation of the form y = mx + b.