Since we know the side length of the square (6), we can calculate its diagonal using pythagoras.
diag d = √(6²+6²) = 6√2 in
The diagonal is also the diameter of the circle! So the radius of the circle is half of that:
radius r = d/2 = 3√2 in
The area of the circle is πr² = π(3√2)² = 18π in²