yu hav 2 boxes ("rektangular solids" in superkrat speek)
area1=16ft wide*21 ft long*4 ft deep=1344 kubik ft
area2=16ft wide*11 ft lone*6ft deep=1056 kubik ft
sum=2400 kubik ft
1 kubik foot=7.480681 gallon
2400 kubik ft=17,953.6344 gallon, or bout 18,000 gal