im not sure with my answer but i think the answer is 3kg...

her empty bucket weights 3kg...

because when you subtract 12kg from 21kg the answer is 9kg...so 9kg is the half weight of the full water in her bucket...and when you multiply it by two the answer is 18kg...

then subtract 18kg from 21kg...so the answer is 3kg...

...im not sure with this...so forgive me if im wrong...