Hmm, it sounds like you must've made an error somewhere. Let's take a look!
First, we'll factor:
x^2 - 6x + 8 = 0
(x - 4)(x - 2) = 0
x = 2, 4
Simple enough, right? Let's complete the square next:
x^2 - 6x + 8 = 0
x^2 - 6x = -8 <<First, I subtracted 8 to the other side to make room for the new c term)
x^2 - 6x + 9 = 1 <<Then, I found the new c by doing (b/2)^2, thus completing the square. This meant (-6/2)^2 which is 9, so I added 9 to both sides.
(x - 3)^2 = 1 <<Next factor the left side.
x - 3 = plus or minus 1 <<Take the square root of both sides.
x = 3 + 1, 3 - 1 << Add over the 3
x = 4, 2
There we go, same as the factored answers! Now let's do the quadratic formula.
x = -(-6) plus/minus sq rt (-6^2-4*1*8) / 2
x = (6 plus/minus sq rt (4)) / 2
x = (6 plus/minus 2) / 2
x = 2, 4
So, your values for x are 2 and 4. Hope this helped!