Answer
The solutions are $-6$ and $6$.
Work Step by Step
$ 3x^{2}+13=121\qquad$ ...add $-13$ to each side.
$3x^{2}+13-13=121-13$
$ 3x^{2}=108\qquad$ ...divide both sides with $3$.
$ x^{2}=36\qquad$ ...take square roots of each side.
$ x=\pm\sqrt{36}\qquad$ ...evaluate $\sqrt{36}$
$x=\pm 6$
