So if you want to have a factored form...
If the zeroes are +4 and +8, you would write
(X-4) (x-8)
Then you would multiply it out to get to quadratic form...
So, (x-4)*(x-8)
X^2 - 12x + 32
Then you would plug it into the formula for axis of symmetry...
X= -b/2a
X=12/2 = 6
X=6