First of all, as coefficients A and B are real, we know that if is a root of an equation, its' complex conjugate is also a root of this equation :
Therefore, is also a root of the equation.
Now, to find the third root we can apply, for example, Vieta's formulas :
To find the coefficients we can either replace in the equation by the roots we've found (which is more direct method), either reapply the Vieta's formulas :
We can even replace in our equation by its' 3 different values and verify that these calculations are correct.
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