correlation coefficient help

Hey, im a first year in leeds uni doing maths and there are some things im starting to get quiet stressed over and they arnt even hard so you'll probably see quite a few of my posts pop up with my exams coming in a few days.

The question im stuck on is:

Let X1 , X2 and X3 be independent random variables with zero mean and variance 1.

Find the correlation coefficient between Y1 = 2(X1) + (X2) and Y2 = (X2) ¡ 2(X3) .

I really dont know where to start and its quite conserning because i dont think its even hard :(.

I beleive that cov(x,y) = E(XY) -E(X)E(Y) but with a zero mean of the variables im unsure how this works. Thanks for all help.