Chapter 5 - Polynomials and Factoring - 5.6 Factoring: A General Strategy - 5.6 Exercise Set - Page 344: 6

$10(c+9)(c-9)$

Factoring the $GCF= 10 $, the given expression, $ 10c^2-810 ,$ is equivalent to \begin{array}{l} 10(c^2-81) .\end{array} Using $x^2-y^2=(x+y)(x-y)$ or the factoring of the difference of 2 squares, then the factored form of the expression above is \begin{array}{l} 10(c+9)(c-9) .\end{array}