The trouble is, there is no one goal in simplifying. You can make it look a little simpler by using base 2 or base 6 instead of base 3, but that's arbitrary. Using base 3, you probably have it about as simple as possible.
Were there specific instructions, either for this problem or a whole group of them? As far as I'm concerned, this is not a very well-designed problem, leaving us so uncertain how far to go. ("Simple" is in the eye of the beholder -- or rather, it depends on what you need to do next with an expression.)
If the 6 had been a 16, it would be easy to know when you were finished! Are you sure you copied it correctly?