Esther starts at one corner of a square field and walks 90 feet along one side to another corner of the field. she turns 90° and walks 70 feet, and then walks straight back to where she started. what is the area of the part of the field she walked around?
a. 3150 ft²
b. 4900 ft²
c. 6300 ft²
d. 8100 ft²