Chapter 4 - Quadratic Functions and Equations - 4-6 Complete the Square - Practice and Problem-Solving Exercises - Page 237: 16

$x=\pm\sqrt 5$

First, we add 15 to both sides to isolate $x^2$ with its coefficient, giving us $3x^2=15$. Then, we divide both sides by 3 to fully isolate $x^2$ to get $x^2=5$. Take the square root of both sides to get $x=\pm\sqrt 5$.