So I have the following equation:

$\displaystyle

r^3 + 4r^2 - 24 = 0

$

I can find the real solution of 2 by using synthetic division and finding which of the multiples of 24 ends up to have no remainder.

But, the back of the book also wants an imaginary number solution and I am lost as to how to find one. Any help would be greatly appreciated.