Covariance of two functions for error propagation

#1
Hi all,

I have two functions:

X=A-B
Y=A+2B

I need to calculate the covariance of X and Y in terms of the (known) variances of A and B as part of a propagation of uncertainty calculation to work out the error on a derived quantity Z=X/Y.

All I can find information on is how to calculate the covariance of sets of measurements rather than two functions such as this. Any ideas on how to calculate cov[X,Y]?

Thanks in advance,

F