What is the axis of symmetry of the function f of x is equal to negative open paren x plus 9 close paren times open paren x minus 21 close paren ?

The zeros of the function are x = -9 and x = 21, from the factors (x + 9) and (x - 21). The axis of symmetry is halfway between the zeros: (-9 + 21) / 2 = 6. Therefore, the axis is x = 6, choice C.

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