Find the zeros of the function f (x) = x2 + 9x + 20 by factoring.

Let a and b be the factors of the expression such that x² + 9x + 20 = (x + a) (x + b). The sum of a and b is the numerical coefficient of the second term and their product is the constant.

a + b = 9; a = 9 - b

ab = 20; (9 - b) (b) = 20

The value of a and b are 4 and 5. The factors are therefore,

(x + 4) (x + 5)

To find the zero, equate the factors to zero which yields an answer equal to x = - 4 and x = - 5.