a)
\frac{2\sqrt3}{7\sqrt2}=\frac{2\sqrt3*\sqrt2}{7\sqrt2*\sqrt2}=\frac{2\sqrt6}{7*2}=\frac{2\sqrt6}{7}
b)
\frac{1}{\sqrt3-\sqrt2}=\frac{1*(\sqrt3+\sqrt2)}{(\sqrt3-\sqrt2)(\sqrt3+\sqrt2)}=
=\frac{\sqrt3+\sqrt2}{(\sqrt3)^2-(\sqrt2)^2}=\frac{\sqrt3+\sqrt2}{3-2}=\sqrt3+\sqrt2