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