use this for
and
sub these and find the solution
Here's a proof of the double angle formula for the sine and cosine functions so that you can better understand them.
sin 2α = sin (α + α)
sin 2α = sin α cos α + cos α sin α
sin 2α = 2 sin α cos α
So, there's two ways to substitute here. The question you have to ask yourself is which one will be more efficacious.