if the problem is to build a sandbox with the length twice the width and there is 24 feet of wood for the perimeter of the sandbox proceed as follows:
Let W = Width
L = Length = 2W
Then, the perimeter is 2W + 2L = 24 = 2W + 2(2W) = 2W + 4W = 6W
or Perimeter = 24 = 6W Then, W = 4 feet and L = 2W = 8 feet