a)
\frac{3}{\sqrt5}=\frac{3*\sqrt5}{\sqrt5*\sqrt5}=\frac{3\sqrt5}{5}
b)
\frac{3\sqrt2}{\sqrt7}=\frac{3\sqrt2*\sqrt7}{\sqrt7*\sqrt7}=\frac{3\sqrt{14}}{7}
c)
\frac{5-2\sqrt5}{3\sqrt5}=\frac{(5-2\sqrt5)*\sqrt5}{3\sqrt5*\sqrt5}=\frac{5\sqrt5-2*5}{3*5}=\frac{5(\sqrt5-2)}{3*5}=\frac{\sqrt5-2}{3}