If the floor is a square, then each side of the floor has length square root of 784 which is 28 feet,
That is because:
The area of a square is its length times its width.
A square has all 4 sides the same so its length is the same as its width.
A = l*w
A = l*l because w = l
A = l^2
784 = l^2
sqrt(784) = l
28 = l