Method of finding all non-trivial homomorphism $\mathbb{Z_{14}}\rightarrow \mathbb{Z_{21}}$

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I have the following question :

Find all non-trivial homomorphism $\mathbb{Z_{14}}\rightarrow \mathbb{Z_{21}}$ I wonder how to solve such problems when the groups are pretty big to check the definition of each possibility.

Any ideas?

Thank you.