Answer
{$25,64$}
Work Step by Step
Since, $u^2-13u+40=0$
Factorize the expression as follows: $(u-8)(u-5)=0$
or, $u=${$5,8$}
Replace $u$ with $\sqrt x$, we have
$\sqrt x=5 \implies x=5^2$
or, $x=25$
and $\sqrt x=8 \implies x=8^2$
or, $x=64$
Hence, our solution set is $x=${$25,64$}