Answer
$\{16,256\}$
Work Step by Step
Substitute $u=x^{1/4},$
$u^{2}-6u+8=0$
... factor: search for factors of 8 with sum -6...
... we find $-2$ and $-4$
$(u-2)(u-4)=0$
$u=2$ or $u=4$
... bring back x
$x^{1/4}=2 \qquad$or$\qquad x^{1/4}=4$
$x=2^{4} \qquad$or$\qquad x= 4^{4}$
$x=16 \qquad$or$\qquad x= 256$
Solution set = $\{16,256\}$
