1 quart = 0.25 gallon.
The Prichard family drinks 2 quarts of milk per day, So they drink 2*0.25 = 0.5 gallon of milk each day.
One week is constituted on 7 days. If they drink a half gallon of milk each day, so they drink 0.5*7 = 3.5 gallons in a week.
So the Prichard Family drinks 3.5 gallons of milk in a week.
Hope this Helps! :)