If two of the zeroes of the polynomial f (x) = x4 - 3x3 - x2 + 9x - 6 are - √3 and √3 then all the zeroes are
√3, - √3, 1, 2
(x + √3)(x - √3) = x2 - 3 will be a factor of f(x).
Now divide f(x) by x2 ¬ 3 to get the quotient x2 - 3x + 2, which has the factors (x - 2)(x - 1).
Hence all the zeros are √3, - √3, 1, 2.