Poles and Zeroes - Z Domain
use the quadratic formula.  you know:

$$ a \, x^2 \ + \ b \, x \ + \ c \ = \ 0 $$

$\implies$

$$ x = \begin{cases}
 \frac{-b \, \pm \sqrt{b^2 - 4 a c}}{2\,a}  \quad \text{if } b^2 \ge 4 a c \\
  \\
 \frac{-b \, \pm j \sqrt{4 a c -b^2}}{2\,a}  \quad \text{if } b^2 &lt; 4 a c\\
\end{cases}$$