please help me solve this elasticity of demand problem.

Consider the following sales function for Brand 1 and answer the questions using the sales function: 21 =135-10P, +5P2 -4P3 + 0.005Y +0.001A,, where 21 = unit sales of Brand 1 (in thousand weekly) Pi = price of Brand 1, currently $20 P2 = price of a second product, Brand 2, currently $25 P3 = price of a third product, Brand 3, currently $15 Y = per capita income in the market, currently $10,000 A = advertizing outlay on Brand 1, currently $50,000. i) Calculate the price elasticity of demand for Brand 1, and write a narrative interpretation of the calculated price elasticity. IS the demand for Brand 1 elastic or inelastic? How do you know? ii) Basing your answer on the calculated price elasticity of demand, if the price of Brand 1 were lowered to $18, ceteris paribus, what is the percentage effect on unit sales of Brand 1? iii) Calculate the cross elastic of demand for Brand 1 with respect to the price of Brand 2 and interpret the meaning of the calculated elasticity. Are Brand 1 and Brand 2 complements or substitutes