c. a person uses 84 gallons of water for showering each week.
Step-by-step explanation:
each day is 12 gallons so just multiply the number of days by 12 to get how many gallons that you use in a set number of days. for the last one, a week is 7 days so if you do 7 x 12 then you get how many gallons a person uses in a week.