Answer 1

According to the question:
Sides of the square are x = 6, x = 9, y = 3 and y = 6.
The vertices of the square are (6, 6), (9, 6), (9, 3) and (6, 3).
And, the vertices of two diagonals are (6, 6), (9, 3) and (9, 6), (6, 3).
Hence, the equation of the circle is \[\left( x - 6 \right)\left( x - 9 \right) + \left( y - 6 \right)\left( y - 3 \right)\] or

\[x^2 + y^2 - 15x - 9y + 72 = 0\]