Define U = X + Y.
(a) E(U) = E(X) + E(Y).
(b) IF X and Y are independent (a big IF but otherwise the question can't be answered) then Var(U) = Var(X) + Var(Y). Therefore sd(U) = ....
(c) You're probably meant to assume that X and Y follow a normal distribution. In which case U does to and has the above parameters. Calculate Pr(U > 1400).