The exercise states that Line passes through and Furthermore, it also states that Line passes through the points and The slopes of both lines are correctly calculated using the Slope Formula. The lines are not parallel because the slopes are not the same. To determine whether or not they are perpendicular, we multiply the slopes. We see that the product of the slopes is not Therefore, the lines are not perpendicular. The error was thinking that, because the slopes are opposite, the lines were perpendicular. The conclusion should be that lines and are neither parallel nor perpendicular.