What is the general form of the equation of a circle with a center at the origin and a radius of 9?
Any general circle centred at
So in this case since the centre is the origin, it implies that
Thus the equation reduces to
General Equation of a circle:
where (a,b) are the coordinates of center and 'r' is the radius
since in you question center lies on origin and the radius is 9
therefore, (a,b) = (0,0) and r=9