cos x- 2sinxcosx is the answer because of the double angle formula for sin which is sinxcosx+cosxsinx which is just multiplication for all trigonometric ratios so these can be rearranged into sinxsinx+cosxcosx due to the commutative property of multiplication and addition. Then just combine like terms to get 2cosxsinx which will replace sin2x so the answer is cosx-2sinxcosx.
