This is a fairly straight forward problem but I don't know how to calculate the covariance given just the variance of X and Y.
Suppose X and Y are independent random variables such that Var(X)=1 and Var(Y)=2. What is the variance of X+2Y-3?
So far, I have
(1^2)var(x) + (2^2)var(y) + 2(1)(2)cov(x, y)
Which comes out to
1 + 8 + 4cov(x, y)
How can I calculate 4cov(x, y)?