Figure20Demand and Supply Curves In the demand curve, the term a is the P-axis intercept. This is the lowest price with zero demand. If the price falls below a, then the quantity demanded rises above zero. Similarly, in the supply curve, the term c is the P-axis intercept. This is the price that is so low, the quantity supplied falls to zero. If the price increases from c, then the quantity supplied rises above zero. The slope of the demand curve is equal to −b and the slope of the supply curve is equal to d. Table 3A-