How to solve this to get the angle?

for 0 < x < 2pisin6x=3sin3x

this is how far i got:

2sin3xcos3x = 3sin3x

2sin3xcos3x/sin3x = 3

2cos3x = 3

cos3x = 3/2

3x = cos-1 (3/2)

But the calculator cant compute, gives me an error. Can someone see where i went wrong?

Thanks!!!!