8.AT.A.2: Use proportional reasoning to identify slope and represent linear relationships. a. Use proportional reasoning to explain why the slope m is the same between any two points on a non-vertical line by showing that the ratios of vertical change to horizontal change between points are equivalent. b. Use this relationship to write equations of lines in the form for lines that pass through the origin, and in the form for lines that include a vertical shift from the origin, where b represents the y-intercept.