This demand curve is a straight line. To draw it you normally make the price axis P vertical and the quantity Q axis horizontal. To draw the line work out the price when the quantity is zero (no demand). This happens when Q=0 and 500P=40000, so divide both sides by 100 by removing two zeroes: 5P=400 then divide through by 5: P=80. So on the P axis mark 80. Although it wouldn't be realistic to have a zero price, nevertheless it will enable you to draw the line. When P=0 Q=40000, so mark a point on the Q axis to indicate 40000. You can calibrate both axes to fit the values, so the Q axis could be measured in units of 1000: 0, 1000, 2000, and so on. Or you could use units of 5000: 0, 5000, 10000, 15000, ..., 40000. and so on. Join these points on the two axis to produce your demand line. You can then read off Q and P values on the line. For example, at Q=20000, P=40.