#### Answer

$\frac{-2\sqrt a}{a^2}$

#### Work Step by Step

$\frac{-2}{\sqrt {a^3}}\longrightarrow$ factor perfect square out of $a^3$
=$\frac{-2}{\sqrt{a^2\times a}}\longrightarrow$ multiplication property of square roots
=$\frac{-2}{\sqrt{a^2}\times \sqrt a}\longrightarrow$ simplify
=$\frac{-2}{a \sqrt a} \longrightarrow$ multiply numerator and denominator by $\sqrt a$
=$\frac{-2}{a\sqrt a}\times \frac{\sqrt a}{\sqrt a}\longrightarrow$ simplify
=$\frac{-2\sqrt a}{a\sqrt {a^2}}\longrightarrow$ simplify
=$\frac{-2\sqrt a}{a\times a}\longrightarrow$ simplify
=$\frac{-2\sqrt a}{a^2}$