Question: A polynomial is given. $(a)$ Factor it into linear and irreducible quadratic factors with real coefficients. $(b)$ Factor it completely into linear factors with complex coefficients.
$x^3 - 5x^2 + 4x - 20$
I factored it using the Rational Zeros Theorem and got the following expression: $(x-5)(x^2 + 4)$. Now, I think this is the answer for the 1st question, but how do I get complex coefficients? I can think of complex factors like $(x-5)(x-2i)(x+2i)$, but complex coefficients?