Solution
For what value of x is the polynomial above x-axis:
The roots of the graph passes through -3 and 3
so the value of x above will be:
The reason why the polynomial is having imaginary roots is
Fundamental Theorem of Algebra we explain that a polynomial will have exactly as many roots as its degree (the degree is the highest exponent of the polynomial). So we know one more thing: the degree is 5 so there are 5 roots in total.
The Fundamental Theorem of Algebra assures us that any polynomial with real number coefficients can be factored completely over the field of complex numbers .