Power series of the form $r \bmod q$

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I am trying exercises of Apostol Introduction to analytic number theory and I am struck on this problem of chapter partitions on page 14.

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I am struck in part (b) as I have no idea on how to deal with the problem as I can put x=x times ${\alpha}^h$ and then product in h=1 to 4 but how to prove that there will be powers of $ \alpha $ along with I's.

Can you please help proving it?