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.
