The general equation of a circle is given by;
[tex] (x-a)^{2} + (y-b)^{2} = r^{2} [/tex]
Where (a, b) is the centre of the circle and r is the radius.
For the given circle, centre is (0,0) and r=9
This
[tex] (x-0)^{2} + (y-0)^{2} = 9^{2} [/tex]
[tex] x^{2} + y^{2} = 9^{2} [/tex]
