Answer:
x^2+y^2=81
Step-by-step explanation:
the equation is
(x-h)^2+(y-k)^2=r^2
since the center is (0,0) you don't even put it in the equation
so all you will have is the x and the y
you leave in the square (^2)
you divide the diameter by 2 to get the radius
9
then you square the radius
81
