18、已知随机变量X1 X2,X3相互独立,且 _(1)sim U(0,6), _(2)sim N((0,2)^2) _(3)sim pi (3),-|||-(2(X)_(1)+(X)_(2),(X)_(3))= __ 若 =(X)_(1)-2(X)_(2)+3(X)_(3), 则 E(Y)= __