$\sqrt{64a}+\sqrt{4a}-\sqrt{121a}=\sqrt{64}\ast \sqrt{a}+\sqrt{4}\ast \sqrt{a}-\sqrt{121}\ast \sqrt{a}=\sqrt{a}(8+2-11)=-\sqrt{a}$

$\sqrt{45}+\sqrt{20}-\sqrt{320}=\sqrt{5\ast 9}+\sqrt{5\ast 4}-\sqrt{5\ast 64}=\sqrt{5}\ast \sqrt{9}+\sqrt{5}\ast \sqrt{4}-\sqrt{5}\ast \sqrt{64}=\sqrt{5}(3+2-8)=-3\sqrt{5}$

$6\sqrt{125a}-2\sqrt{80a}+3\sqrt{180a}=6\sqrt{5a\ast 25}-2\sqrt{5a\ast 16}+3\sqrt{5a\ast 36}=6\ast \sqrt{5a}\ast \sqrt{25}-2\ast \sqrt{5a}\ast \sqrt{16}+3\ast \sqrt{5a}\ast \sqrt{36}=\sqrt{5a}(6\ast 5-2\ast 4+3\ast 6)=\sqrt{5a}(30-8+18)=40\sqrt{5a}$