Compute the total amount pumped out in the last three days:
210 gallons/day x 3 days = 630 gallons
Add the volume pumped out (removed) to the volume today:
Volume today = 6600 gallons
Volume pumped out = 630 gallons
Volume 3 days ago = 6600 gallons + 630 gallons = 7230 gallons
Thus the volume three days ago was 7230 gallons.
The tank now has 6,600 gallons of water. During the last three days 210 gallons of water was pumped out each day.
The total volume of water pumped out is equal to 210*3 = 630 gallons.
This gives the volume of the tank 3 days ago as 630 + 6600 = 7230 gallons.