A=$\sqrt{12-3\sqrt{7}}$ - $\sqrt{12+3\sqrt{7}}$
=$\sqrt{12-2\sqrt{15,75}}$ - $\sqrt{12+2\sqrt{15,75}}$
=$\sqrt{10,5-2\sqrt{15,75}+1,5}$ - $\sqrt{10,5+2\sqrt{15,75}+1,5}$
=$\sqrt{(\sqrt{10,5}-\sqrt{10,5})^{2}}$ -$\sqrt{(\sqrt{10,5}+\sqrt{10,5})^{2}}$
=|$\sqrt{10,5}$ - $\sqrt{1,5}$| - |$\sqrt{10,5}$ + $\sqrt{1,5}$|
=$\sqrt{10,5}$ - $\sqrt{1,5}$ - ($\sqrt{10,5}$ + $\sqrt{1,5}$)
=$\sqrt{10,5}$ - $\sqrt{1,5}$ - $\sqrt{10,5}$ - $\sqrt{1,5}$
=-2$\sqrt{1,5}$
=-√6