\[y = \frac{1}{x}.\]
\[x \in \lbrack - 3; - 1\rbrack:\]
\[x = - 3 \Longrightarrow y = \frac{1}{- 3} = - \frac{1}{3}.\]
\[x = - 1 \Longrightarrow y = \frac{1}{- 1} = - 1.\]
\[y \in \left\lbrack - 1;\ - \frac{1}{3} \right\rbrack.\]