Find the exact values of sin 2u, cos 2u, and tan 2u using the double angle formulas.:

cos u = -2/3, pi/2 < u < pi

So I know that the formulas are:

sin 2u = 2 sin u cos u

cos 2u = cos^2 u - sin^2 u

tan 2u = (2 tan u) / (1 - tan^2 u)

Now I am having trouble figuring out what sin u is. I believe that it is

sqrt(3), but I am not sure.

If I am right, the first formula would be:

sin 2u = (2) (sqrt3) (-2/3)

but I'm not sure how to multiply a square root with a fraction.

Any help is appreciated.