The wall area would be (9 ft) [ (2) (21 ft) + (2) (22 ft) ]
Simplifying, A = Area = 9(42+44) ft^2 = 774 square feet.
You did not share info regarding how many square feet of all a gallon container will cover. Let us for example purposes only suggest that each gallon will cover 400 square feet.
Then:
774 sq ft 1 gal
------------- * --------------- * 2 coats = 3.87 gal. Round this up to 4 gal.
1 400 sq ft
