1-√a     _   3-√a 2√a-4       3√a-6

[latex] frac{1-sqrt{a}}{2sqrt{a}-4} - frac{3-sqrt{a}}{3sqrt{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{3-3sqrt{a}-6+2sqrt{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} }{3sqrt{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]