You almost had it right! remember the equation of a circle is
(x - h)^2 + (y - k)^2 = r^2. the center is (h,k) and r is the radius.
(x - 0)^2 + (y - 0)^2 = 5^2
x^2 + y^2 = 25 <<< your answer
hope that helps, God bless!
F(4) is equivalent to 28 because when you plug in 4 for the x, you get 28.
