a: \(=\dfrac{\sqrt{3}+2+\sqrt{3}-2}{-1}\cdot\dfrac{-1}{2\sqrt{3}}=-1\)
b: \(=\dfrac{\sqrt{x}+2+\sqrt{x}-1}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-1\right)}\cdot\dfrac{x-4}{2\sqrt{x}}\)
\(=\dfrac{\left(2\sqrt{x}+1\right)\left(\sqrt{x}-2\right)}{2\sqrt{x}\left(\sqrt{x}-1\right)}\)