Question 57257
Just fill in i in your function and check if it becomes 0. 
Make use of the property that i² = -1

P(i) = i³ - i² -i*(i²) -20i + i*i + 20i

     = -i + 1 + i - 20i - 1 + 20i

     = 0

Check for yourself that P(-i) is NOT equal to zero (same mehod as above)

This does not contradict the Conjugate Pair Theorem, because not all the coefficients of P(x) are real.

Greets,
Scriptor