`a)`

`(2x+4)(x-\sqrt{2})=0`

`=>`\(\left[ \begin{array}{l}2x+4=0\\x-\sqrt{2}=0\end{array} \right.\) 

`=>`\(\left[ \begin{array}{l}x=-2\\x=\sqrt{2}\end{array} \right.\) 

Vậy `S={-2;\sqrt{2}}`

`b)`

`\sqrt{x^2+4x+4}=3`

`<=>\sqrt{(x+2)^2}=3`

`<=>|x+2|=3`

`=>`\(\left[ \begin{array}{l}x+2=3\\x+2=-3\end{array} \right.\) 

`=>`\(\left[ \begin{array}{l}x=1\\x=-5\end{array} \right.\) 

Vậy `S={-5;1}`