[tex]\frac{12}{\sqrt{7}-1}=\frac{12(\sqrt{7}+1)}{(\sqrt{7}-1)(\sqrt{7}+1)} =\frac{12(\sqrt{7}+1)}{(\sqrt{7}) ^{2}-1^{2}}=\frac{12(\sqrt{7}+1)}{7-1}=\frac{12(\sqrt{7}+1)}{6}=2(\sqrt{7}+1)[/tex]