#### Answer

{$\pm 1, \pm 2$}

#### Work Step by Step

Since, $u^2-5u+4=0$
Factorize the expression as follows: $(u-4)(u-1)=0$
or, $u=${$1,4$}
Replace $u$ with $x^2$, we have
$x^2=1$ and $x^2=4$
This implies that $x=\pm 1, \pm 2$
Hence, our solution set is $x=${$\pm 1, \pm 2$}