Alright, here is how you do this.
We know that if the housewife uses 2 buckets a day during the course of 12 days, that means, all in all, she used 6 buckets during those twelve days.
This means that if she uses 3 buckets a day, she will only have enough charcoal for 2 days, because 6/3 is 2.
Therefore, your final answer is 2 days
I hope this helped! Comment if you need me to explain it further or have any questions regarding it! :)