$$-y^3+22y+6441 = 0$$

$$ \begin{matrix}y_1 = 19 & y_2 = -\dfrac{ 19 }{ 2 }+\dfrac{\sqrt{ 995 }}{ 2 }i & y_3 = -\dfrac{ 19 }{ 2 }-\dfrac{\sqrt{ 995 }}{ 2 }i \end{matrix} $$

$ \color{blue}{ -x^{3}+22x+6441 } $ is a polynomial of degree 3. To find zeros for polynomials of degree 3 or higher we use Rational Root Test.

The Rational Root Theorem tells you that if the polynomial has a rational zero then it must be a fraction $ \dfrac{p}{q} $, where p is a factor of the trailing constant and q is a factor of the leading coefficient.

The factor of the leading coefficient ( -1 ) is 1 .The factors of the constant term (6441) are 1 3 19 57 113 339 2147 6441 . Then the Rational Roots Tests yields the following possible solutions:

$$ \pm \frac{ 1 }{ 1 } , ~ \pm \frac{ 3 }{ 1 } , ~ \pm \frac{ 19 }{ 1 } , ~ \pm \frac{ 57 }{ 1 } , ~ \pm \frac{ 113 }{ 1 } , ~ \pm \frac{ 339 }{ 1 } , ~ \pm \frac{ 2147 }{ 1 } , ~ \pm \frac{ 6441 }{ 1 } ~ $$

Substitute the POSSIBLE roots one by one into the polynomial to find the actual roots. Start first with the whole numbers.

If we plug these values into the polynomial $ P(x) $, we obtain $ P(19) = 0 $.

To find remaining zeros we use Factor Theorem. This theorem states that if $\frac{p}{q}$ is root of the polynomial then this polynomial can be divided with $ \color{blue}{q x - p} $. In this example:

Divide $ P(x) $ with $ \color{blue}{x - 19} $

$$ \frac{ -x^{3}+22x+6441 }{ \color{blue}{ x - 19 } } = -x^{2}-19x-339 $$

Polynomial $ -x^{2}-19x-339 $ can be used to find the remaining roots.

$ \color{blue}{ -x^{2}-19x-339 } $ is a second degree polynomial. For a detailed answer how to find its roots you can use step-by-step quadratic equation solver.

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