[tex]15\sqrt{80}:(3\sqrt{5})+6\sqrt{288}:(-2\sqrt{2})-21\sqrt{338}:(-7\sqrt{2}) =\\ \\ = 15\sqrt{4^2\cdot 5}:(3\sqrt{5})+6\sqrt{12^2\cdot 2}:(-2\sqrt{2})-21\sqrt{13^2\cdot 2}:(-7\sqrt{2})\\ \\ = 15\cdot 4\sqrt{5}:(3\sqrt{5})+6\cdot 12\sqrt{2}:(-2\sqrt{2})-21\cdot 13\sqrt{2}:(-7\sqrt{2})\\ \\ = 60\sqrt{5}:(3\sqrt{5})+72\sqrt{2}:(-2\sqrt{2})+3\cdot 13\\ \\ = 20-36+39\\ \\ = \bf{23}[/tex]
