The roots of polynomial $ p(r) $ are:
$$ \begin{aligned}r_1 &= -2.1568\\[1 em]r_2 &= -6.7005\\[1 em]r_3 &= 8.8573 \end{aligned} $$Polynomial $ -r^3+64r+128 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.