# Bounds for Fourier coefficients of cusp forms

I've asked the background question here, which still left unanswered. Now I have a more precise question. In my homework I've been asked to prove that $$\left| \sum_{1\leq n \leq N} a_f (n)e^{2\pi i n \alpha}\right| \leq c_f N^{k\over 2}\log N$$ for any $f \in S_k$ where $f(\tau) = \sum\limits_{n=1}^\infty a_f (n)q^n$, any real $\alpha$ and any $N \geq 10$.
That one I have proved. Now I have to deduce that we have the same bound for the coefficients restricted to any arithmetic progression - that is for any $1 \leq q \in \mathbb Z$ and $a \pmod q$ , we have: $$\left| \sum_{1 \leq n \leq N , n \equiv a \pmod q} a_f (n)\right| \leq c_f N^{k \over 2} \log N .$$

Can someone give me a hint on that one? I know that coefficients may change signs and I don't really know when, so a subset of them may sum to something larger.

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