Find all the real zeros of the polynomial $$P(x)=x^4 + x^3 - 13 x^2 - x + 12.$$
Its real zeros are $x_1=$ , $x_2=$ , $x_3=$ and $x_4=$ with $x_1\le x_2\le x_3\le x_4$.
Note: If the polynomial has only three real zeros, input them at $x_1$, $x_2$ and $x_3$; if the polynomial has only two real zeros, input them at $x_1$ and $x_2$; if the polynomial has only one real zero, input it at $x_1$.