What is the axis of symmetry of the function f(x) =-(x+9)(x-21)

f(x) =-(x+9)(x-21)

Manipulating algebrically,
We get,
f(x) =-(x+9)(x-21)
      = -(x^2 -21x + 9x - 189)
      = -(x^2 -12x - 189)
      = -x^2 + 12x + 189
Axis of symmetry = -b/2a
plugging the values,
x = -12/-2
   = 6