Chapter 11 - Polynomial and Rational Functions - 11.1 Power Functions and Proportionality - Exercises and Problems for Section 11.1 - Skill Refresher - Page 439: S6

$\pm \frac{1}{10}$

First, divide both sides of the equation by $5$ to get $$x^{-2}=100$$ Next, use the fact that $a^{-n}=\frac{1}{a^n}$ to get $$\frac{1}{x^2}=100$$ Take the reciprocal of both sides to get $$x^2=\frac{1}{100}$$ Finally, take the square root of both sides to get $$x=\sqrt{\frac{1}{100}}=\pm \frac{1}{10}$$