Which are real zeroes of this function? x3 + 2x2 - 9x - 18

Try putting x = - 2:-

f (-2) = - 8 + 2 (-2) ^2 - 9 (-2) - 18 = - 8 + 8 + 18 - 18 = 0

so one zero is - 2

as the last term is - 18 it is also worth trying to see is 3, - 3 or 9 or - 9 are roots (by the Rational Roots Theorem)

in fact f (3) = 0 and f (-3) = 0 also

So the real zeroes are - 2,-3 and 3.