The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.3805\\[1 em]x_2 &= -4.3805 \end{aligned} $$The solutions of $ -3x^2-12x+5 = 0 $ are: $ x = -2-\dfrac{\sqrt{ 51 }}{ 3 } ~ \text{and} ~ x = -2+\dfrac{\sqrt{ 51 }}{ 3 }$.
You can use step-by-step quadratic equation solver to see a detailed explanation on how to solve this quadratic equation.