The perimeter of a rectangle is twice the sum of length and width, so the sum of length and width is half the perimeter, 10 3/4 ft.
The width will be the difference between this and the length, hence 5 1/2 ft.
The area is the product of length and width.
A = length×width
A = (5.25 ft)×(5.5 ft) = 28.875 ft²
The area of the sandbox is 28 7/8 ft².