Answer
$f^{-1}(x)=-\frac{3}{x} $
See graph.
Work Step by Step
Step 1. $f(x)=-\frac{3}{x} \Longrightarrow y=-\frac{3}{x} \Longrightarrow x=-\frac{3}{y} \Longrightarrow y=-\frac{3}{x} \Longrightarrow f^{-1}(x)=-\frac{3}{x} $
Step 2. Check $f(f^{-1}(x))=-\frac{3}{-\frac{3}{x}}=x$ and $f^{-1}(f(x))=-\frac{3}{-\frac{3}{x}}=x$
Step 3. See graph.