Answer
$\left\{-4, 4\right\}$
Work Step by Step
Divide $3$ on both sides of the equation to obtain:
$x^2=16$
Take the square root of both sides to obtain:
$x = \sqrt{16}
x = \pm 4$
Thus, the solution set of the given equation is $\left\{-4, 4\right\}$.
