Ihave to solve x^2-6x+8=0 by factoring, quadratic equation, and completing the square. i did this and got different answers for all of them. me understand how to solve using each of the ways.

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!