Any slope that is parallel to the y-axis, or simply just a vertical line would be considered D) Undefined. The reason for this is in a function, if you were to perform the vertical line test (drawing a straight vertical line through the function to see if any points overlap), no points would overlap. However, in graphs that are not functions, points overlap each other vertically. Another way to remember your slopes is by imagining a hill. When you are going up the hill, you have a positive slope (for example, a slope of 3). When you are going down the hill, you are having a negative slope (for example,a slope of -3). When you are on the flat part of the hill, or having a horizontal line on your graph, you are having no slope, or a slope of zero. When you are going straight down a hill/edge of cliff, your slope is undefined.