Working on a task where I´m supposed to find the roots of $P(z) = z^4 - 6z^3 + 11z^2 - 2z - 10$.
One root is given, $z_0 = 2 - i$.
First, it´s clear another root is $z_1 = 2 + i$.
I know I´m supposed to calculate another polynomial by using $(z - z_0)(z - z_1)$.
The solution tells me that equals $z^{2} - 4z + 5$. But I can´t see how that´s possible? Could someone please explain how this is calculated?