Square feet of rectangle = 50 x 10 = 500 square feet.
The perimeter of the rectangle is 50 + 50 + 10 + 10 = 120 feet.
Change to a square: 120/4 = 30
The square would have a side length of 30 feet.
Area of the square = 30 x 30 = 900 square feet.
The square is 900. - 500 = 400 square feet more.