[tex]a)\;\;\; \frac{12}{ \sqrt{3}} =\frac{12 \sqrt{3}}{3} =\boxed{4 \sqrt{3}} \\ b)\;\;\; \frac{21}{ \sqrt{14}} =\frac{21 \sqrt{14}}{14} =\boxed{\frac{3 \sqrt{14}}{2}} \\ c)\;\;\; \frac{-28}{4 \sqrt{7}} =\frac{-28 \sqrt{7}}{4*7}=\frac{-28 \sqrt{7}}{28} =\boxed{-\sqrt{7}} \\ d)\;\;\; \frac{ \sqrt{2} }{ \sqrt{3}} =\frac{ \sqrt{2} \sqrt{3}}{ 3} =\boxed{\frac{ \sqrt{6}}{ 3}} \\ e)\;\;\; \frac{ \sqrt{6} }{ 2\sqrt{2}} = \frac{ \sqrt{2} \sqrt{3}}{ 2\sqrt{2}} =\boxed{\frac{ \sqrt{3}}{ 2}} [/tex]
[tex]f)\;\;\; \frac{5\sqrt{3} }{ 4\sqrt{5}} = \frac{ 5\sqrt{3} \sqrt{5}}{ 4\;*\;5} =\boxed{\frac{ \sqrt{15}}{ 4}}[/tex]