Question

What is the length and width of a 864 square feet

Answer 1

The area doesn't tell you the dimensions. There are an infinite number of different answers, all with the same area but different dimensions.
Here are a few:
1 ft by 864 ft
2 ft by 432 ft
3 ft by 288 ft
4 ft by 216 ft
8 ft by 108 ft
16 ft by 54 ft
24 ft by 36 ft
27 ft by 32 ft.

The area of each of these is 864 square feet.