$b)Q=(\dfrac{\sqrt{14}-\sqrt{7}}{1-\sqrt{2}}+\dfrac{\sqrt{15}-\sqrt{5}}{1-\sqrt{3}}):\dfrac{1}{\sqrt{7}-\sqrt{5}}$
$Q=(\dfrac{-(\sqrt{7}-\sqrt{14})}{1-\sqrt{2}}-\dfrac{\sqrt{5}-\sqrt{15}}{1-\sqrt{3}}).(\sqrt{7}-\sqrt{5})$
$Q=(\dfrac{-\sqrt{7}(1-\sqrt{2})}{1-\sqrt{2}}-\dfrac{\sqrt{5}(1-\sqrt{3})}{1-\sqrt{3}}).(\sqrt{7}-\sqrt{5})$
$Q=(-\sqrt{7}-\sqrt{5}).(\sqrt{7}-\sqrt{5})$
$Q=-(\sqrt{7}+\sqrt{5}).(\sqrt{7}-\sqrt{5})$
$Q=-(7-5)$
$Q=-2$
