Chapter 4 - Quadratic Functions and Equations - 4-6 Complete the Square - Practice and Problem-Solving Exercises - Page 237: 17

$x=\pm\sqrt 8$

First, we add 40 to both sides to isolate $x^2$ with its coefficient, giving us $5x^2=40$. Then, we divide both sides by 5 to fully isolate $x^2$ to get $x^2=8$. Square root both sides to get $x=\pm\sqrt 8$.