do u know your double angle formulae?
sin2x = 2sinxcosx = 2cosxsinx
in this case, x = 3x
so 2cos3xsin3x = sin2(3x) = sin6x
If say the sin3x was replaced with sin3y, you'd have to use your products to sums so its sometimes a bit confusing but if the variables are the same, its double angle...