Answer:
3,500 feet²
Explanation:
The average home size in United States in 1970 was 1,400 square feet.
The average size was about 
 times great in 2004.
 times great in 2004.
The difference in size of home = 

 = 

 = 3,500 square feet.
The average home size was 3,500 square feet in 2004.