Answer
$$\sqrt {3} $$
Work Step by Step
Recall that the definition $\sqrt{-a} = i\sqrt{a}$ must be applied BEFORE the other rules regarding radicals, therefore,
$\frac{\sqrt {-30}}{\sqrt{-10}} = \frac{i\sqrt {30}}{i\sqrt{10}}$
$\sqrt \frac{30}{10}$
$$\sqrt 3$$