1-√a     _   3-√a 2√a-4       3√a-6

[latex] \frac{1-\sqrt{a}}{2\sqrt{a}-4} - \frac{3-\sqrt{a}}{3\sqrt{a}-6}= \frac{1-\sqrt{a}}{2(\sqrt{a}-2)} - \frac{3-\sqrt{a}}{3(\sqrt{a}-2)}= \frac{3(1-\sqrt{a})-2(3-\sqrt{a})}{6(\sqrt{a}-2)} =\\ \\ =\frac{3-3\sqrt{a}-6+2\sqrt{a}}{6(\sqrt{a}-2)} =\frac{-3-\sqrt{a}}{6(\sqrt{a}-2)} =-\frac{3+\sqrt{a}}{6(\sqrt{a}-2)} [/latex]

[latex] \frac{1- \sqrt{a} }{2 \sqrt{a} -4} - \frac{3- \sqrt{a} }{3\sqrt{a} -6}= \frac{1- \sqrt{a} }{2( \sqrt{a} -2)} - \frac{3- \sqrt{a} }{3(\sqrt{a} -2)} = \frac{1- \sqrt{a} }{2( \sqrt{a} -2)} - \frac{3- \sqrt{a}}{3(\sqrt{a} -2)} = \\ = \frac{3(1- \sqrt{a}) }{6( \sqrt{a} -2)} - \frac{2(3- \sqrt{a}) }{6(\sqrt{a} -2)} = \frac{3(1- \sqrt{a} )-2(3- \sqrt{a}) }{6( \sqrt{a} -2)} = \frac{3-3 \sqrt{a}-6+2 \sqrt{a} }{6 \sqrt{a}-12 } = \\ = \frac{-3- \sqrt{a} }{6 \sqrt{a}-12 } =- \frac{ 3+\sqrt{a} }{6 \sqrt{a} -12} [/latex]