You need to provide a screenshot or recreation of the rest of the problem's information.
3 gallons because there are 4 quarts in a gallon and 12/4=3
Since a serving is 1/2 cup and he needs 72 servings you can multiply 72 by 1/2 and find out that he needs to make 36 cups of punch. Two cups is a pint so he needs (36 / 2) 18 pints of juice. He already has the 8 pints of pineapple juice so 18 - 8 leaves 10 pints of orange juice.
