Salut.
[tex]\frac{\sqrt{42}-\sqrt{18} }{\sqrt{6} } =\frac{\sqrt{42}-3\sqrt{2}}{\sqrt{6}} =\\ \\=\frac{(\sqrt{42}-3\sqrt{2})*\sqrt{6} }{\sqrt{6}*\sqrt{6} } =\frac{\sqrt{252}-3\sqrt{12}}{6} =\\ = \frac{6\sqrt{7}-6\sqrt{3}}{6} =\frac{6(\sqrt{7}-\sqrt{3})}{6}=\sqrt{7}-\sqrt{3}[/tex]