\(\sqrt{45}+\sqrt{125}=\sqrt{9}⋅\sqrt{5}+\sqrt{25}⋅\sqrt{5}=3\sqrt{5}+5\sqrt{5}\text{.}\)

\(3\sqrt{5}+5\sqrt{5}=8\sqrt{5}\text{.}\)

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

\(-3\sqrt{28}=-3⋅\sqrt{4}⋅\sqrt{7}=-3⋅2⋅\sqrt{7}=-6\sqrt{7}\text{.}\)

\(3\sqrt{45}-2\sqrt{125}=3⋅\sqrt{9}⋅\sqrt{5}-2⋅\sqrt{25}⋅\sqrt{5}\text{.}\)

\(3⋅3⋅\sqrt{5}-2⋅5⋅\sqrt{5}=9\sqrt{5}-10\sqrt{5}=-1\sqrt{5}\text{.}\)

\(\sqrt{\frac{16}{81}}={\sqrt{16} \over \sqrt{81}}=\frac{4}{9}\text{.}\)