Let us assume f (x) = x⁴ + x³ ‒ 11x² ‒ 9x + 18

As 3 and – 3 are the zeros of the given polynomial therefore each one of (x + 3) and (x - 3) is a factor of f (x)

Consequently, (x – 3) (x + 3) = (x² – 9) is a factor of f (x)

Now, on dividing f (x) by (x² – 9) we get:

f (x) = 0

(x² + x – 2) (x² – 9) = 0

(x – 1) (x + 2) (x – 3) (x + 3) = 0

∴ x = 1 or x = - 2 or x = 3 or x = - 3

Hence, all the zeros of the given polynomial are 1, -2, 3 and -3