If X1, X2, and X3 are independent and have the means 4, 9, abd 3 and the variances 3, 7, and 5, find the mean and the variance of

(a) Y = 2X1 - 3X2 +4X3;

(b) Z = X1 + 2X2 - X3

Also, could someone tell me how to write the subscript using proper mathematical notation on this website? Thanks!