It's a matter of solving it backwards. You start
with three equations with x equaling to them:

        x=-2,  x=i.   x=-i

Get 0 on the right of each of these because that
would have been the next to last step of solving
the equation: 

       x+2=0, x-i=0, x+i=0

Before that was the zero factor principle, so to 
reverse that we multiply all three equation together

       (x+2)(x-i)(x+i) = 0

           (x+2)(x²-i²) = 0

Then use the fact that i² = -1

            (x+2)(x²-(-1)) = 0

               (x+2)(x²+1) = 0
               
                x³+x+2x²+2 = 0

Putting the terms in descending order:

                x³+2x²+x+2 = 0

Edwin