Agallon of latex paint can cover 500 square feet. maria has a rectangular room whose dimensions are 21 feet by 22 feet, with 9-foot ceilings. how many gallon containers of paint does she need to buy to paint two coats on each wall?

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