Answer:
{-√6, -1, 1, √6}
Explanation:
You want the roots of the quartic 3x^4-21x^2+18.
Factors
We can find the roots by factoring to linear factors:
= 3(x^4 -7x^2 +6)
= 3(x^2 -6)(x^2 -1) . . . . . . . 6 = (-1)(-6) are factors with a sum of -7
= 3(x -√6)(x +√6)(x -1)(x +1) . . . . . a^2 -b^2 = (a -b)(a +b)
The roots of the quartic are ±√6 and ±1.