I know it has something to do with the double identities, but I don't know where to start. Thanks in advance.
If x is a positive acute angle and , what is ?
You need to know that sin(2x) = 2 sinx cos x (this is one of the famous double angle formulas).
You're given sin x so now all you have to do is calculate cos x. I assume this will be similar to problems you have previously done.
Then substitute both into the above formula.