Your trying to dilute it by 66% which is going to be impossible without losing flavor.
Option 1: Buy the same juice @0mg and mix it and accept 9mg.
Option 2: Buy 2 of the same juices @0mg and mix it to your target of 6mg.
Option 3: Salvage the juice by buying VG+flavoring and try matching/cloning the flavoring.
Option 4: Accept it if your sub ohming/using high watts stop and vape it at low watts and a around a 1.8ohm coil/15-20watts.
Option 5:Accept your lost and trash it.
Sorry.