The estimated market demand for good X is

Q̂ = 70 - 3.5P - 0.6M + 4P_Z

where Q̂ is the estimated number of units of good X demanded, P is the price of the good, M is income, and P_Z is the price of related good Z. (All parameter estimates are statistically significant at the 1 percent level.)
a. Is X a normal or an inferior good? Explain.
b. Are X and Z substitutes or complements? Explain.
c. At P = 10, M = 30, and P_Z = 6, compute estimates for the price (Ê), income (Ê_M), and cross-price elasticities (Ê_XZ).