Answer:
x = - 7, x = 3
Step-by-step explanation:
To find the zeros let y = 0, that is
- x² - 4x + 21 = 0 ( multiply through by - 1 )
x² + 4x - 21 = 0
Consider the factors of the constant term (- 21) which sum to give the coefficient of the x- term (+ 4)
The factors are + 7 and - 3, since
7 × - 3 = - 21 and 7 - 3 = + 4, thus
(x + 7)(x - 3) = 0
Equate each factor to zero and solve for x
x + 7 = 0 ⇒ x = - 7
x - 3 = 0 ⇒ x = 3
