Answer:
k = -7
Step-by-step explanation:
Given
x² - x - 42 = (x + k)(x + 6)
Required
Find k
x² - x - 42 = (x + k)(x + 6)
Expand the left hand side
x²-7x + 6x - 42 = (x + k)(x+ 6)
Factorize
x(x - 7) + 6(x- 7) = (x + k)(x+ 6)
Further factorize
(x - 7)(x + 6) = (x + k)(x + 6)
By comparison
x + k = x - 7
Subtract x from both sides
k = -7
