As the square has two sides tangent to the circle, the lenght of the side will be de diameter of the circle.
The area of a circle, which is pi*r², in this case is 9a²pi², given that a=6.
So:
Once r=31.89,, the diameter will be 2*r= 63.78
The area of the square is (63.78)² = 4,067.89u²