The radius of the inner circle is sqrt(9) = 3. The radius of the
outer circle is therefore 3*3 = 9.
The equation for the outer circle is then x^2 + y^2 = 9^2 = 81.
(This is an application of Pythagoras' theorem a^2 + b^2 = c^2 in a
right-angled triangle by the way)