`(1-\sqrtx)^2=9` `(x>=0)`
`<=>(1-\sqrtx)^2=(\sqrt9)^2`
`<=>1-\sqrtx=+-3`
`<=>`\(\left[ \begin{array}{l}1-\sqrt x=3\\1-\sqrt x=-3\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}\sqrt x=-2\text{(vô lí)}\\\sqrt x=4\end{array} \right.\)
`=>\sqrtx=(\sqrt4)^2`
`<=>x=16` (TM)
Vậy: `S={16}`