Chapter 9 - Section 9.1 - Solving Quadratic Equations by the Square Root Property - Exercise Set - Page 631: 9

$x=\pm2$

Using the properties of equality, the given equation, $ 5x^2=20 ,$ is equivalent to \begin{array}{l}\require{cancel} x^2=\dfrac{20}{5} \\\\ x^2=4 .\end{array} Taking the square root of both sides, then the solutions of the given equation are \begin{array}{l}\require{cancel} x=\pm\sqrt{4} \\\\ x=\pm\sqrt{(2)^2} \\\\ x=\pm2 .\end{array}