Answer:
12 cups
Step-by-step explanation:
convert the values to cups-
6 gallons is 24 quarts
24 quarts is 48 pints
48 pints is 96 cups
and for the other value you do the same
21 quarts is 42 pints
42 pints is 84 cups
finally you subtract the 84 cups from the 96 cups to get 12 cups
