4. Let the random variables x and y represent the number of vehicles that arrive at two separate street corners during a certain 2-minute period in the day. The joint distribution is f ( x, y ) = - 4(+y) '16 for x = 0,1,2,?), and y = 0,1,2, [?) Consider Z = X+Y, the sum of two. Find E(Z) and Var (Z)