LHS = cos (2x + x)
cos 2x.cosx-sin2x.sinx
(2cos^2 x - 1)(cosx) - (2sinxcosx)(sinx)
2cos^3x -cosx -2sin^2xcosx
cosx(2cos^2x - 1 -2sin^2x)
cosx (2cos^2x - 1 - 2(1-cos^2x)
cosx(2cos^2x - 1 -2 + 2cos^2x)
cosx (4cos^2x -3)
= 4cos^3x - 3cosx
phew that was effort. but i hope it makes sense