I'm assuming that the square is inscribed in a circle and that is the radius that you are looking for.

(1) S is side of the square so S^2=72 and s=sqrt(72)=8.49

(2) Draw the square. Next draw a circle around the square so that the corners of the square touch the circle.

(3) Draw a point in the center of the square

(4) Draw a line from the center of the square to the top of one side. Draw a line from the center of the square to the bottom of the same side.

(

(5) You now have a triangle. The two sides connected to the center are the radius and the angle at the center point is a 90 degree angle

(6) A triangle with a 90 degree angle is a right triangle and its base is a side of the square that we calculated in (1) 8.49

(7) Now we can use the Pythagorean theorum H^2=A^2 + B^2

A and B are both radii of the circle and H = 8.49

(8) So we have 8.49^2=R^2+R^2

.(9) 72 = 2R^2

(10) 36 = R^2

(11) R=sqrt(36) = 6

(1) S is side of the square so S^2=72 and s=sqrt(72)=8.49

(2) Draw the square. Next draw a circle around the square so that the corners of the square touch the circle.

(3) Draw a point in the center of the square

(4) Draw a line from the center of the square to the top of one side. Draw a line from the center of the square to the bottom of the same side.

(

(5) You now have a triangle. The two sides connected to the center are the radius and the angle at the center point is a 90 degree angle

(6) A triangle with a 90 degree angle is a right triangle and its base is a side of the square that we calculated in (1) 8.49

(7) Now we can use the Pythagorean theorum H^2=A^2 + B^2

A and B are both radii of the circle and H = 8.49

(8) So we have 8.49^2=R^2+R^2

.(9) 72 = 2R^2

(10) 36 = R^2

(11) R=sqrt(36) = 6