Recall that complex multiplication is like that for polynomials, and that i2 is -1. So if you have a polynomial ax + b, what can you multiply it by so that you get an answer with no x term; i.e., of the form cx2 + d?
It may help dredge up your memories from algebra class if you think about trying ot factor something of the form a2x2 - b2.
If that isn't enough, there is one more hint.