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The degree of a polynomials tells you how many zeros there are. So f(x) has three zeros. You were only given 2 zeros so one is missing.
If a polynomial with real coefficients has a complex zero, then the complex conjugate of that zero will also be a zero. A complex number in standard form is:
a + bi
The complex conjugate of this is:
a - bi or a + (-bi)
f(x) has a complex zero: -4i. So the complex conjugate of -4i will be our missing zero. -4i in standard form is:
0 + (-4i)
Its complex conjugate will be:
0 - (-4i) or 0 + 4i or just 4i
So the only zero that you were not given was 4i.