Verified answer

Here's some background:

http://www.mathsisfun.com/algebra/completing-squar...

For your one we have:

-6(x-4)^2 + 48 = 0

Or:

6(x-4)^2 = 48

Divide through by 6:

(x-4)^2 = 8

x - 4 = +/- SqRoot(8)

x = 4 +/- SqRoot(8)

x = 4 +/- 2 SqRoot(2)

Check here:

http://www.wolframalpha.com/input/?i=-6%28x-4%29^2...

So now we have: