Answer
$-\dfrac{1}{a^3}$
Work Step by Step
RECALL:
(1) $a^{-m} = \dfrac{1}{a^m}$
(2) $\dfrac{a^m}{a^n} = a^{m-n}$
(3) $\left(\dfrac{a}{b}\right)^m=\dfrac{a^m}{b^m}$
(4) $(ab)^m = a^mb^m$
Use rule (1) above to obtain:
$-a^{-3} = -\left(\dfrac{1}{a^3}\right)=-\dfrac{1}{a^3}$
