a) [tex]4 \sqrt{7} - 4 \sqrt{7} -6 + \sqrt{12} = \sqrt{12} -6= \sqrt{12} - \sqrt{36} = 1- \sqrt{3} [/tex]
b) [tex] 5\sqrt{15} -3 \sqrt{15} -6+ 4- 4\sqrt{15} =-2 \sqrt{15} -2[/tex]
c) [tex]4 \sqrt{3}+ 2\sqrt{2} - \sqrt{8} -2 \sqrt{12} =4 \sqrt{3} +2 \sqrt{2} - 2\sqrt{2} - 4\sqrt{3} =0[/tex]
d) [tex]2 \sqrt{6} -9- 2\sqrt{2} +6=2 \sqrt{6} -3- 2\sqrt{2} [/tex]