Question 236848
Hint: Recall that all complex zeros come in conjugate pairs. So -3i is also a zero. Now if {{{x=3i}}}, then {{{x^2=-9}}} and {{{x^2+9=0}}}. Now just perform polynomial long division to simplify {{{(x^4-x^3+7x^2-9x-18)/(x^2+9)}}}. This will basically factor {{{x^4-x^3+7x^2-9x-18}}} into two quadratics, which are easily solved by the quadratic formula.