The perimeter will only give you the sum of length and width, so in this case, since the perimeter is twice this sum, length plus width=119". The maximum area is when length=width=x, so the sum is 2x=119, making x=59.5" and the area, x^2, is 3540.25 sq in.
As an example, let the length be 69" and the width 50" to give us a perimeter of 238", but an area of 3450sq in.