Answer
$x=6-\sqrt 3,6+\sqrt 3$
Work Step by Step
When taking the square root of a number, you get a positive and a negative answer.
$-5(x-6)^2-15=-30$
$-5(x-6)^2=-15$
$(x-6)^2=3$
$x-6=\sqrt 3$
$x=6+\sqrt 3$
or
$x-6=-\sqrt 3$
$x=6-\sqrt 3$.
So
$x=6-\sqrt 3,6+\sqrt 3$.