This would depend upon how many floors the building has, and what shape/size each floor is :D
To answer your question honestly, though, the area of a rectangle is length * width. As your question already hints, the answer is to simply multiply the two numbers together!
21ft * 30ft = 630 square feet
21 feet times 19 feet = 399 square feet
Square feet measure area, and yards measure length. If you mean FEET in 21 yards, the answer is 63 (3 times 21). If you mean square feet in 21 SQUARE YARDS, the answer is 189 (three times three times 21).
21 feet by 21 feet is a square having an area of 441 square feet.
14.4375 square feet.
61" x 21" = 8.89583 square feet.
21*21 = 441 square feet.21*21 = 441 square feet.21*21 = 441 square feet.21*21 = 441 square feet.
21 feet by 21 feet is 441 square feet which converts to 49 square yards.
21 square feet.
21 x 4 = 84 square feet.
21*3 = 63 square feet.
Just multiply the 21 by the 12 and you have 252 square feet.
21 x 21 = 441 square feet
21 feet times 19 feet = 399 square feet
14 x 21 = 294 square feet.
21 x 12 = 252 square feet.
21 square yards = 189 square feet.
189 square feet. (multiply square yards by 3 to get square feet)