Chapter 5 - Polynomials and Factoring - 5.2 Factoring Trinomials of the Type x2+bx+c - 5.2 Exercise Set - Page 317: 22

$(x-7)(x+6)$

The 2 numbers whose product is $ac= 1(-42)=-42 $ and whose sum is $b= -1 $ are $\left\{ -7,6 \right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $ x^2-x-42 ,$ then the factored form is \begin{array}{l} x^2-7x+6x-42 \\\\= (x^2-7x)+(6x-42) \\\\= x(x-7)+6(x-7) \\\\= (x-7)(x+6) .\end{array}