Chapter 5 - Polynomials and Factoring - 5.2 Factoring Trinomials of the Type x2+bx+c - 5.2 Exercise Set - Page 317: 60

$(b+10c)(b-2c)$

The 2 numbers whose product is $ac= 1(-20)=-20 $ and whose sum is $b= 8 $ are $\left\{ 10,-2 \right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $ b^2+8bc-20c^2 ,$ then the factored form is \begin{array}{l} b^2+10bc-2bc-20c^2 \\\\= (b^2+10bc)-(2bc+20c^2) \\\\= b(b+10c)-2c(b+10c) \\\\= (b+10c)(b-2c) .\end{array}