Chapter 11 - Quadratic Functions and Equations - 11.1 Quadratic Equations - 11.1 Exercise Set - Page 706: 10

$c=\pm2\sqrt{2}$

$x^2=a$ implies $x=\pm\sqrt{a}$ (the Square Root Principle).Therefore, the solutions to the given equation, $ c^2-8=0 ,$ are \begin{array}{l}\require{cancel} c^2=8 \\\\ c=\pm\sqrt{8} \\\\ c=\pm\sqrt{4\cdot2} \\\\ c=\pm\sqrt{(2)^2\cdot2} \\\\ c=\pm2\sqrt{2} .\end{array}