Find all rational zeros of the polynomial $$P(x)= 2 x^4 + 2 x^3 - 13 x^2 - x + 6,$$ and then find the irrational zeros, if any.
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 rational zeros, input them at $x_1$, $x_2$ and $x_3$; If the polynomial has only two rational zeros, input them at $x_1$ and $x_2$; if the polynomial has only one rational zero, input it at $x_1$. Give EXACT answers. No decimals.