No, I havenít calculated the total weight on the floor. The way to do that would have been 16x4x3.5 = 224 cu.ft.
Then 224x62.22 lb/cu.ft = 13,937 lbs. Total
Therefore, 13937 divided by the area of the floor (64) =218 lb/sq ft.
So based on the last part of your explanation, the lateral force pushing out from a point 2 feet deep 2/3.5 x 218 = 125 pounds/sq ft.
Those are the correct units as far as the math is concerned, but not the correct units for for the answer to the problem. The answer has to be in pounds.