The bottom line is: when given an interval and a polynomial check the low value and high value. If one is positive and the other negative then there must be a zero within that interval.
3x^3+5x^2-6x+5 with x=-3 is 3*-27 + 5*9 + -6*-3 + 5 = -
3x^3+5x^2-6x+5 with x=-1 is -3 + 5 + 6 + 5 = +
Since one value is above y=0 and the other below y=0, the polynomial must have a zero within that interval.
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