This example has a function of a function of a function.

Let `u = 2x` and `v = cos 2x`

So we can write `y = v^3` and `v = cos\ u`

`(dy)/(dx)=(dy)/(dv)(dv)/(du)(du)/(dx)`

`=3v^2(-sin u)(2)`

`=3(cos^2 2x)(-sin 2x)(2)`

`=-6\ cos^2 2x\ sin 2x`