`@\color{blue}{\24th12}`

`\sqrt{9x+27}+4\sqrt{x+3}-3/4 \sqrt{16x+48}=0` `(x>=-3)`

`<=>\sqrt{9(x+3)}+4\sqrt{x+3}-3/4 \sqrt{16(x+3)}=0` 

 `<=>3\sqrt{x+3}+4\sqrt{x+3}-3 \sqrt{x+3}=0` 

`<=>(3+4-3)\sqrt{x+3}=0` 

`<=>4\sqrt{x+3}=0` 

`<=>\sqrt{x+3}=0` 

`<=>x+3=0`

`<=>x=-3`

Vậy `S={-3}`