Answer
$x=8-\sqrt {47},8+\sqrt {47}$
Work Step by Step
When taking the square root of a number, you get a positive and a negative answer.
$-0.25(x-8)^2+18=6.25$
$-0.25(x-8)^2=-11.75$
$(x-8)^2=47$
$x-8=\sqrt {47}$
$x=8+\sqrt {47}$
or
$x-8=-\sqrt {47}$
$x=8-\sqrt {47}$.
So
$x=8-\sqrt {47},8+\sqrt {47}$.