Draw the square's diagonals; these are also the diameters of the circle. Label a half-diameter as "r", the radius.
What is the area of a circle with radius r?
By its nature, the diameters of the circle split the square into four isosceles triangles with base-angles of 45 degrees. What is the length of one of the equal-length sides of one of these right triangles? What then is the length, in terms of r, of a side of the square, being the hypotenuse of the triangle?
What is the area of a square with this side length?
Since the square is inside the circle, subtract the square's area from the circle's area to find an expression for what you need.
Then differentiate with respect to time, etc, etc.
If you get stuck, please reply showing how far you have gotten. Thank you!