I hope this helps you
12x/2+24/2
6x+12
-6 ( -x-2)
1 week = 7 days
1 gallon = 16 cups ⇒ 1 cup = 1/16 gallon
1.5 cups = 1.5 · 1/16 gallon = 15 · 1/160 gallon = 3/32 gallon
1.5 cups per day = 3/32 gallons per day = 7 · 3/32 gallons per week
= 21/32 gallons per week
4.3125 is the answer to that equation