a: \(=\left(2\sqrt{3}+6\sqrt{3}\right)\cdot\dfrac{\sqrt{3}}{2}-5\sqrt{6}\)
\(=8\sqrt{3}\cdot\dfrac{\sqrt{3}}{2}-5\sqrt{6}=12-5\sqrt{6}\)
b: \(=2\sqrt{7}-\sqrt{7}-2\sqrt{3}+2\sqrt{21}\)
\(=\sqrt{7}-2\sqrt{3}+2\sqrt{21}\)
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