#### Answer

$x=-7.6,21.6$

#### Work Step by Step

When taking the square root of a number, you get a positive and a negative answer.
$3(x-7)^2+8=647.48$
$3(x-7)^2=639.48$
$(x-7)^2=213.16$
$x-7=14.6$
$x=21.6$
or
$x-7=-14.6$
$x=-7.6$.
So
$x=-7.6,21.6$.