Answer
$A=18$ sq.units
Work Step by Step
The diagonal of the square equals the diameter of the circle.
From the equation, r=3, diameter = 6
The sides a of the square and the diagonal d satisfy Pythagorean theorem
$a^{2}+a^{2}=d^{2}$
$2a^{2}=6^{2}$
$a^{2}=\displaystyle \frac{36}{2}=18$
$a^{2}$ is the area of the square....
$A=18$ sq.units