Expectation of Discrete Random Variables?

For the next 3 questions:

X is a random variable with μx = 8.7 and σx = 18.5

Y is a random variable with μy = 14.9 and σy = 21.0

X and Y are independent.

1. What is the mean of X + Y?

2. What is the standard deviation of X + Y?

3. What is the standard deviation of X - Y?

1. E(X + Y) = E(X) + E(Y) = 8.7 + 14.9 = 23.6

23.6 / 2 = 11.8

2. Var (X + Y) = Var (X) + Var (Y) -> COV = 0 since independant

= 18.5^2 + 21.0^2 = 342.25 + 441 = 783.25

SD = Sqrt(783.25) = 27.99

3. I get a negative number following the same steps I used above, and obviously you can't square root a negative number. Am I doing something wrong?

Anyways, am I doing these questions properly? Any help or suggestions would be AWESOME.