Thanks to sympy, this task would be easy for it.

from sympy import solve, Symbol
x = Symbol('x')
a, b, c, d = (1, 0, -0.8, -0.14) #(0.2 - 1.0 = -0.8), (-0.7 * 0.2 = -0.14)
print(solve(a * x ** 3 + b * x ** 2 + c * x + d))