I am having trouble with this problem...It goes as follows:

Find the exact value of:

sin(2x), cos(2x), and tan(2x) if cos(x) = -4/5, [pi]/2 <x<[pi].

It gives me a hint saying( sin2x=2sinxcosx and cos(2x)=2cos^2 (X-1)

I have no idea how to do this Anyone care to help?