What you've done so far is fine, now you just have to evaluate the values of x...
sin(3x)cos(6x)-cos(3x)sin(6x)=-0.9
I solve like this
sin(3x)cos(6x)-cos(3x)sin(6x)=-0.9
sin(3x-6x)=-0.9
sin(-3x)=-0.9
-sin(3x)=-0.9
sin(3x)=0.9
but the answer are 0.373+2π/3k and 0.6742π/3k
π is pi.
help please? am I doing the right way?