The Slope (also called Gradient) of a straight line shows how steep a straight line is. To calculate the slope, you need two points from the graph. Then, get the difference of the y and x values of the points and divide y by x. We do as follows:
slope = y2-y1 / x2 - x1 = 1.4 - 7.2 / 18.1 + 5 = -0.2511