Answer: 1 1/12 cups of brown sugar.
Explanation:
2/3 i equal to 8/12
3/4 is equal to 9/12
for every 8/12 of white sugar, you have to use 9/12 of brown sugar, that is 1/12 more than the white sugar.
12/12 is equal to one cup, so one cup of white sugar is 12/12, so if the brown sugar is 1/12 more than the white sugar, you have to put 13/12 or 1 1/12 cups of brown sugar. Hope this Helped:)
(sorry if I'm wrong)