EXPLANATION
Let's see the facts:
Center of the circle---> Origin = (0,0)
Pass through point ---> (9,0)
As we can see in graph, the radius of the circle is equal to r=9
With this given information, the equation standard form will be:
(x-h)^2 + (y-k)^2 = r^2
With value of h=0 and k=0, equation will be:
x^2+y^2 = 9^2
[ANSWER] x^2 + y^2 = 81