Answer:
x = 6
Step-by-step explanation:
Take f(x) = -(x+9)(x-21) and multiply it out. We get:
f(x) = -[x^2 + 9x - 21x - 189], or
f(x) = -x^2 + 12x + 189
This is a quadratic equation. The coefficients are a = -1, b = 12 and c = 189.
The equation of the axis of symmetry is x = -b/(2a), which here equals:
x = -12 / [2*(-1)], or x = 6.
