\(\sqrt{8}=\sqrt{4}⋅\sqrt{2}=2\sqrt{2}\text{.}\)

\(-5\sqrt{18}=-5⋅\sqrt{9}⋅\sqrt{2}=-5⋅3⋅\sqrt{2}=-15\sqrt{2}\text{.}\)

\(\sqrt{7\frac{1}{9}}=\sqrt{\frac{64}{9}}={\sqrt{64} \over \sqrt{9}}=\frac{8}{3}=2\frac{2}{3}\text{.}\)

\(\sqrt{\frac{8}{25}}={\sqrt{8} \over \sqrt{25}}={\sqrt{8} \over 5}=\frac{1}{5}\sqrt{8}=\frac{1}{5}⋅2⋅\sqrt{2}=\frac{2}{5}\sqrt{2}\text{.}\)