According to the Fundamental theorem of algebra, every non-zero, single-variable, degree n polynomial with complex coefficients has, counted with multiplicity, exactly n complex roots.
Therefore, the degree of our polynomial will have a degree greater or equal to 3.Moreover, our polynomial could be represented in form of "(x-x_1)(x-x_2)(x-x_3)" if all the roots are real. But the third root is a complex number, so it is a complex root of an "x^2-ax+b" with "a^2-4b<0" . We know that the second root of such an equation will have form 3-9i, so the polynomial in question has to have 4 roots and degree 4. The form of the polynomial is "(x-8)(x+14)(x-(3+9i))(x-(3-9i)) = x^4-58x^2+1212x-10080"
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