If the square fits the circle exactly, then its diagonal is equal to the diameter of the circle. Since the side of the square has a length of cm, then it's diagonal have the length of cm. Using the circle's area we can find the diagonal of the square, as shown below:
Since the diameter of the circle is the same as the diagonal of the square, then:
The easiest way to do this is for you to subtract all the sides from the total perimeter since the perimeter is all of the sides added and your remainder is your answer