Question 751826
The side of the square = 84/4 = 21cm
The perimeter of the rectangle = 84/3 = 28cm
Perimeter = 2 lengths + 2 breadths 
            2 lengths + 2 breadths = 28 cm
            2 x 8 cm  + 2 breadths = 28 cm
                        2 breadths = 28 cm - 16
                          breadth  = 6 cm

Therefore difference between the breadth of the
rectangle and the side of the square is:

            21cms - 6cms =  15 cms