Perimeter of square = 72 cm

Perimeter of rectangle

=\( \Large \frac{72}{2} \)=36 cm

=> \( \Large 2\times (l+b) \)=36

[l and b are dimension of rectangle]

\( \Large 2\times (12+b) \)=36

=> 12 + b = 18

b=18-12=6cm

And side of square = \( \Large \frac{72}{4} \)=18

Difference between breadth of rectangle and side of the square

= 18 - 6 = 12 cm.