`\sqrt{42}-\sqrt{30}+\sqrt{21}-2\sqrt{6}-\sqrt{15}-2\sqrt{3}`
`=\sqrt{42}-\sqrt{30}+\sqrt{21}-\sqrt{24}-\sqrt{15}-\sqrt{12}`
`=\sqrt{3}.\sqrt{14}-\sqrt{3}.\sqrt{10}+\sqrt{3}.\sqrt{7}-\sqrt{3}.\sqrt{8}-\sqrt{3}.\sqrt{5}-\sqrt{3}.\sqrt{4}`
`=\sqrt{3}.(\sqrt{14}-\sqrt{10}+\sqrt{7}-\sqrt{8}-\sqrt{5}-\sqrt{4})`
`=\sqrt{3}.[(\sqrt{14}-\sqrt{10}-\sqrt{8}-\sqrt{4})+\sqrt{7}-\sqrt{5}]`
`=\sqrt{3}.[\sqrt{2}.(\sqrt{7}-\sqrt{5}-\sqrt{4}-\sqrt{2})+\sqrt{7}-\sqrt{5}]`
`=\sqrt{3}.[\sqrt{2}.(\sqrt{7}-\sqrt{5}-2-\sqrt{2})+\sqrt{7}-\sqrt{5}]`