`\sqrt{x+3}+\sqrt{6-x}=\sqrt{(x+3)(6-x)}+3(-3<=x<=6)`
`<=>x+3+6-x=(x+3)(6-x)+9+6\sqrt{(x+3)(6-x)}`
`<=>9=9+(x+3)(6-x)+6\sqrt{(x+3)(6-x)}`
`<=>(x+3)(6-x)+6\sqrt{(x+3)(6-x)}=0`
`<=>\sqrt{(x+3)(6-x)}(\sqrt{(x+3)(6-x)}+6)=0`
`<=>\sqrt{(x+3)(6-x)}=0`
`<=>x=-3\or\x=6`
Vậy `S={-3,6}`