\[\boxed{\mathbf{531\ (531).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[1)\ \sqrt{9a} + \sqrt{25a} - \sqrt{49a} =\]
\[= 3\sqrt{a} + 5\sqrt{a} - 7\sqrt{a} = \sqrt{a};\]
\[2)\ \sqrt{64b} - \frac{1}{6}\sqrt{36b} = 8\sqrt{b} - \sqrt{b} =\]
\[= 7\sqrt{b}\]
\[= 0,4\sqrt{c} - 1,2\sqrt{c} + 0,3\sqrt{c} =\]
\[= - 0,5\sqrt{c};\]
\[= 4\sqrt{m} + 10\sqrt{m} - 1,8\sqrt{m} =\]
\[= 12,2\sqrt{m}\]