Answer:
x³ - 5x² + 81x - 405 = 0
Step-by-step explanation:
Complex roots occur in conjugate pairs.
Thus given x = - 9i is a root then x = 9i is also a root
The factors are then (x - 5), (x - 9i) and (x + 9i)
The polynomial is the the product of the roots, that is
f(x) = (x - 5)(x - 9i)(x + 9i) ← expand the complex factors
= (x - 5)(x² - 81i²) → note i² = - 1
= (x - 5)(x² + 81) ← distribute
= x³ + 81x - 5x² - 405, thus
x³ - 5x² + 81x - 405 = 0 ← is the polynomial equation
