A wire is in the shape of a circle of radius 21 cm.  It is bent to form a square.  The side of the square is

Circumference of circle $=2\pi r$

$=2\times \frac{22}{7}\times 21$

$=132cm$

Circumference of circle $=$  Perimeter of square

$132=4a$

$a=\frac{132}{4}=33$

Side of square $=33cm$ .