Answer
$x=-4+\sqrt -7,-4+\sqrt 7$
Work Step by Step
$3(x+4)^{2}=21$
$(x+4)^{2}=7$
$x+4=\sqrt 7$
$x+4=\sqrt -7$
$x=-4+\sqrt -7,-4+\sqrt 7$
Divide both sides. Then take the square root of both sides. Then isolate $x$ from both of the resulting equations to get two answers.