Answer:
x = - 7, x = 3
Step-by-step explanation:
To find the x- intercepts let f(x) = 0 , that is
x² + 4x - 21 = 0
Consider the factors of - 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, then
x² + 4x - 21 = 0
(x + 7)(x - 3) = 0 ← in factored form
Equate each factor to zero and solve for x
x + 7 = 0 ⇒ x = - 7
x - 3 = 0 ⇒ x = 3
