If $4^x + 4^{-x} = 34$, then $2^x + 2^{-x}$ is equal to...? I am having trouble with this:

If $4^x + 4^{-x} = 34$, then what is $2^x + 2^{-x}$ equal to? 

I managed to find $4^x$ and it is:
$$4^x = 17 \pm 12\sqrt{2}$$
so that means that $2^x$ is:
$$2^x = \pm \sqrt{17 \pm 12\sqrt{2}}.$$
Correct answer is 6 and I am not getting it :(. What am I doing wrong?

$4^{x}+4^{-x} = 34$
$2^{2x}+2^{-2x} = 34$
Let $z= 2^x$.
