(i) Well the joint density is the product of marginal densities, since they are independent. So can you do it now?
(ii) Var(X-Y) = Var(X) + Var(-Y) = Var(X) + Var(Y) = 9 + 1 = 10
The first equality is justified because X and Y are independent.
Why is the second equality justified?
(iii) $\displaystyle E(X^2) = Var(X) + (E(X))^2 = 9 + 2^2 = 13$
(iv) $\displaystyle E(X+Y) = E(X) + E(Y) = 2 - 3 = -1$