Answer: She should buy 21 gallons of fruit juice and 7 gallons of soda.
Step-by-step explanation:
Let x be the number of gallons of fruit juice and y be the number of gallons of soda.
Then,


Multiply 3 on the both sides of equation (1), we gte

Using elimination method, subtract (3) from (2), we get

Substitute y=7 in (1), we get

Hence, She should buy 21 gallons of fruit juice and 7 gallons of soda.