Answer:
x^2 + y^2 = 81
Explanation:
Mathematically, the equation of a circle can be represented by ;
(x-h)^2 + (y-k)^2 = r^2
where (h,k) is the center of the circle which is the origin in this case. The origin has the coordinates (0,0)
the radius here is 9 units
Thus, the equation of the circle would be;
(x-0)^2 + (y-0)^2 = 9^2
= x^2 + y^2 = 81