Answer
$$r=4,\:r=-2$$
Work Step by Step
The quadratic formula states:
$$x_{1,\:2}=\frac{-b\pm \sqrt{b^2-4ac}}{2a}$$
Thus, solving the equation, we find:
$$ r_{1,\:2}=\frac{-\left(-2\right)\pm \sqrt{\left(-2\right)^2-4\cdot \:1\left(-8\right)}}{2\cdot \:1} \\ r=4,\:r=-2$$