[tex] \frac{\sqrt{2}+ \sqrt{8}+x}{3}=2 \sqrt{2} \Leftrightarrow \frac{ \sqrt{2}+ 2 \sqrt{2}+x}{3}=2 \sqrt{2} \Leftrightarrow \frac{3 \sqrt{2}+x}{3}=2 \sqrt{2} \Leftrightarrow \\ \\ \Leftrightarrow 3 \sqrt{2}+x=6 \sqrt{2} \Rightarrow \boxed{x= 3 \sqrt{2}} .[/tex]
