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

The axis of symmetry is x =

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

= - (x² - 21 x + 9 x - 189) =

= - x² + 21 x - 9 x + 189 =

= - x² + 12 x + 189

This is a quadratic function and its axis of symmetry is:

x = - b / 2a, where: a = - 1 and b = 12

x = - 12 / 2· (-1) = - 12 / ( - 2) = 6

Answer: x = 6