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Hey,
the problem is formulated INCORRECTLY.
The correct formulation is THIS:
A polynomial function with real coefficients is given.
Find all remaining zeros. Degree: 3 Zeros: -1,i.
The theory which was not explained to you is THIS:
If the polynomial with real coefficients has a complex root (a+ib),
then it has the conjugate complex number (a-ib) as a root, too.
In your case, since you are given that the complex number "i" is the zero, it implies that the complex number "-i" is the root to your polynomial, too.
Thus the list of zeroes in your case is {-1, i, -i}.
Answered, solved and completed.