6) \(-\(\sqrt{18}\)+2\(\sqrt{72}\)-\(\sqrt{98}\)=

\(\sqrt{18}=3\sqrt2\), \(\sqrt{72}=6\sqrt2\), \(\sqrt{98}=7\sqrt2\).

\(-3\sqrt2+2\cdot6\sqrt2-7\sqrt2=(-3+12-7)\sqrt2=2\sqrt2\).

Ответ: \(2\sqrt2\).