Need help answering this practice problem.

6. The RVs X and Y are independent and have the PDFs given by fx (x) = a exp(-ax) , x 20 0, otherwise and fy (y) = ja exp(-ay) , y 20 0, otherwise where a # B. (a) Compute E[X+Y] and E[XY]. (b) What is the CDF of max {X,Y}? (c) What is the PDF of max {X,Y}? (d) Compute the CDF and the PDF of Z = X + Y