If is a factor of the polynomial, it means that
is a root of the polynomial.
In fact, if a polynomial has a root
(i.e.
), then the polynomial is divisible by
In your case, you know that the polynomial is divisible by , so
is a solution.
Let's evaluate and set it to zero:
We can check the answer: if the polynomial becomes
And we're claiming that is a root of this polynomial: