I think the best strategy would be to multiply x²+x+1 by other polys and see if you can get another polynomial out that only has 3 nonzero terms. One thing you know about these polys that they must have a constant term of 1. Do you see why?
Also, you could try making a multiplication grid:
x²
x
1 1
1 ax ...
Basically, you want to see what kinds of polynomials you can get out of this multiplication.
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