Identities simplify expression

[disneychannel]

I have been trying to do this problem for a long time, and still can not do it. I know the answer is sin2x, but I have no idea how to do it:

write expression as sine, cosine, or tangent of an angle

sin3xcosx - cos3xsinx

THANKS!

 

[Dick]

Use the sine addition formula, sin(a-b)=sin(a)cos(b)-cos(a)sin(b). Line that up with your expression and figure out what a and b should be.