See the attachment. They are much easier to solve if you set up a spreadsheet template and then copy that template for each problem. The inputs would be the two given points for each graph. Problem 3's slope is undefined, since it is a vertical line and the run is 0, no change in x. Since slope is "Rise/Run," this is dividing by zero, a No-No for mathematicians, as well as us chemists and other mortals.