Answer
$\{-3, 1\pm2i \}$
Work Step by Step
Step 1. List all possible rational zeros as
$\frac{p}{q}=\pm1, \pm3, \pm5, \pm15 $
Step 2. Use synthetic division to find one zero $x=-3$ as shown in the figure.
Step 3. With the quotient $x^2-2x+5=0$, we have $x=\frac{2\pm\sqrt {-16}}{2}=1\pm2i$
Step 4. We have the solution set as $\{-3, 1\pm2i \}$
