Suppose that the sales function for a product L has been estimated as: QL=7-1.6PL-.2PC+.8PS+.0005Y+.0004A Where, QL is the quantity of product L demanded, in thousand per month. PL is the price of product L, which is currently $15 PC is the price of product C, which is currently $25; PS is the price of product S, which is currently $20; Y is the level of per capita income, which is currently $20,000; and A is monthly advertising, which is currently $15,000. Express the functional relationship between the amount of sales of L (QL) and the price of L (PL) alone, which is the demand function of L