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**A Beautiful Mind** Find All Zeros.

$\displaystyle f(x) = 3x^3-20x^2+7x+30$

Alright, I applied the Rational Zero Theorem and thought I found these as possibilities:

1,3,10,15,2,30

____________

1,3

Plus or minus: 1, 3, 10, 15, 2, 30, 1/3, 10/3, 5, 2/3.

I tested them all and didn't get anything as far as I know. Now I don't know what to do. I think I'm supposed to find ALL zeros, so that could mean imaginary as well. I know there's at least three zeros, right? How'd I go about working this one out?