#### Answer

$x=\pm\sqrt 5$

#### Work Step by Step

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 $x=\pm\sqrt 5$.