3. Using regression analysis on data from a field experiment, the demand curve for a product is estimated to be 

Q_X^d=1,200-3P_X-0.1P_Z

 where Pz = $300. 

(a) What is the own price elasticity of demand when Px = $140? Is demand elastic or inelastic at this price? What would happen to the firm's revenue (increase or decrease) if it decided to charge a price below $140?

(b) What is the cross-price elasticity of demand between good X and good Z when Px = $140? Are goods X and Z substitutes or complements?

For both (a) and (b) you have to calculate the elasticity at a point on the demand curve. Look at example on page 89-90 of the textbook.