6.
[tex]a) \ 5\sqrt{7}-2\sqrt{7} = 3\sqrt{7}\\\\b) \ \sqrt{5}\cdot\sqrt{3} = \sqrt{5\cdot3} = \sqrt{15}\\\\c) \ \sqrt{18}:\sqrt{2} = \sqrt{18:2} = \sqrt{9} = 3\\\\d) \ 2\sqrt{3}+6\sqrt{3} = 8\sqrt{3}[/tex]
