box% sage
┌────────────────────────────────────────────────────────────────────┐
│ SageMath version 8.2, Release Date: 2018-05-05                     │
│ Type "notebook()" for the browser-based notebook interface.        │
│ Type "help()" for help.                                            │
└────────────────────────────────────────────────────────────────────┘
sage: f=x^3-2*x^2-9*x+18
sage: f
x^3 - 2*x^2 - 9*x + 18
sage: f.subs({x: x+2/3})
1/27*(3*x + 2)^3 - 2/9*(3*x + 2)^2 - 9*x + 12
sage: f.subs({x: x+2/3}).expand()
x^3 - 31/3*x + 308/27
sage: