Question

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

Answer 1

x = 6

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.