Intermediate Algebra for College Students (7th Edition)

{$49$}
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 $x=${$49$}