we know that
the equation of a circle is
(x-h)²+(y-k)²=r²
where (h,K) is the center and
r is the radius
in this problem
the center is (0,0) and the radius is √81-----> r=9 units
the graph intersect the y-axis when x=0
so
x²+y²=81--------> 0+y²=81--------> y=(+/-)√81------> y=(+/-) 9
the points are
(0,9)
and
(0,-9)
