Please help understand how to solve the following type of question

c. The estimated market demand for a product X is Q = 70 - 3.5P - 0.61 + 4P., Where Q is the estimated demand of product X (in number of units), P is the price of product X, I is the income, and P, is the price of related product. (All independent variables are statistically significant). Answer the following questions: i . Is X a normal product or an inferior product? Explain in 15-20 words. ii. Are X and Z products substitutes or complements? Explain in 15-20 words