Given f (x) = x² - 6x + 8 and g(x) = x³ - 2x² - 9x - 18, find the domain of f over g of x.

a) What is the domain of f over g of x?
a. {x ∈ ℝ}
b. {x ∈ ℝ | x ≠ -2, -3, 3}
c. {x ∈ ℝ | x ≠ 3, -3}
d. {x ∈ ℝ | x ≠ 4}