Answer:
9/16 Cup of Brown Sugar
Explanation:
Convert the 3/8 Cups of Brown Sugar to 6/16 to make division easier.
So for every 2/3 Cup of White Sugar you use 6/16 Cup of Brown Sugar.
So 1/3 a Cup of White Sugar would use 3/16 a Cup of Brown Sugar.
So 3/3 or 1 Cup of White Sugar would use 9/16 a Cup of Brown Sugar