а) 6√5 + 2√45 − √80

Упростим корни: \(\sqrt{45}=3\sqrt{5}\), \(\sqrt{80}=4\sqrt{5}\).

\(6\sqrt{5}+2\cdot3\sqrt{5}-4\sqrt{5}=(6+6-4)\sqrt{5}=8\sqrt{5}\).

Ответ: \(8\sqrt{5}\).