Why am I wrong, the answer should be -10 / 6-

The others already explained that you divided by 4 instead of 2. Alternatively, starting from the first line you wrote one can recognize the difference of two squares x^2 + 4x + 4 = (x+2)^2 and 64=8^2. From there the polinomial factorises into

(x+2-8)(x+2+8) = (x - 6)(x + 10)

and you can read the solutions 6 and -10.
\-b/(2a) for this problem is not -4/4
x = - p/2 + ... not  x = - p/4 + ... as you did.
I have no idea what formula you used... But it isn't

(-b +- sqrt(b^2 - 4ac))
------------------------
2a
It’s -10, and 6

-b +/- sqrt(b^2 -4ac) / 2a

-4 +/- sqrt(256) / 2

-4 +/- 16 / 2

(-4+16)/2= 6
(-4-16)/2 =-10
Please, for the love of everyone's sanity, ESPECIALLY your own, don't use mathematical symbols in ambiguous ways. That / looks like an abbreviated fraction; use a comma for a list, and ideally also {} to denote solution sets, so:

{6, -10}

You may think "oh, context will make it clear, it's fine," and you may be right, this time. But later, when you have more to keep track of, that kind of loose notation will cause you untold, and completely unnecessary, pain.
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