Answer
$-2$
Work Step by Step
$\because y=\log_a x \text{ is equivalent to } x= a^y$
$\therefore y = \log_3 \left(\dfrac{1}{9}\right) \text{ means } 3^y=\dfrac{1}{9}$
Note that:
$\dfrac{1}{9}= \dfrac{1}{3^2} = 3^{-2}$
$\therefore 3^y=3^{-2}$
Use the rule $a^m=a^n \implies m=n$ to obtain:
$y=-2$
Therefore,
$\log_3 \left(\dfrac{1}{9}\right) = \boxed{-2}$
