Question 706040
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The other tutor made an error.

Since -3i is a root so is its conjugate 3i.  
That's because -3i is 0-3i and its conjugate is 0+3i or just 3i.

So the roots are

 x=-3i,   x=3i,  x=-2,   x=4

x+3i=0, x-3i=0, x+2=0, x-4=0

(x+3i)·(x-3i)·(x+2)·(x-4) = 0

        (x²-9i²)(x²-2x-8) = 0
      [x²-9(-1)](x²-2x-8) = 0
          (x²+9)(x²-2x-8) = 0

Multiply that out and collect terms and get:

         x<sup>4</sup>-2x³+x²-18x-72 = 0

Edwin</pre>