The best way is to recognise that, if x = 5 is a root, then x − 5 = 0 , and ditto for the other two roots. So we have x − 5 , x − i , x + i all equalling zero. To find our polynomial, we just multiply the three terms together: ( x − 5 ) ( x − i ) ( x + i )
= ( x 2 − i x − 5 x + 5 i ) ( x + i )
= x 3 + i x 2 − i x 2 − ( i 2 ) x − 5 x 2 − 5 i x + 5 i x + 5 i 2