Let θ=arctan 6x, then sin θ=6x/sqrt(1+36x2) and cos θ=1/sqrt(1+36x2).
Let Φ=arccos x, then sin Φ=sqrt(1-x2) and cos Φ=x
Now use these values to compute sin θ cos Φ - cos θ sin Φ.
This expression will be messy (at best) and may be able to be collected and simplified.