#### Answer

a) $\frac{\sqrt 21}{7}$
b) $9+3\sqrt5$

#### Work Step by Step

a) $\frac{\sqrt 3}{\sqrt 7}$ = $\frac{\sqrt 3}{\sqrt 7} \times \frac{\sqrt 7}{\sqrt 7}$
= $\frac{\sqrt 3\times\sqrt 7}{\sqrt 7\times\sqrt 7}$
= $\frac{\sqrt 21}{7}$
b) $\frac{12}{3-\sqrt 5}$ = $\frac{12}{3-\sqrt 5}$ $\times$ $\frac{3+\sqrt5}{3+\sqrt 5}$
= $\frac{36+12\sqrt5}{9-5}$
= $\frac{36+12\sqrt5}{4}$
= $9+3\sqrt5$