Answer
{$49$}
Work Step by Step
Since, $u^2-4u-21=0$
Factorize the expression as follows: $(u-7)(u+3)=0$
or, $u=${$-3,7$}
Replace $u$ with $\sqrt x$, we have
$\sqrt x=-3$ which has no solution.
or, $\sqrt x=7 \implies x=7^2$
and $x=49$
Hence, our solution set is $x=${$49$}