Pumping Lemma proof, need help with splitting xyz word, choosing pumping length and proving its not regular

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I have the following language i want to prove is not regular by using the pumping lemma but not sure on what to use as the xyz and the pumping length. can i get a point in the right direction for how to prove it is not regular. {z^7n c^5 c^n+7 c^m | n,m >= 0} i have tried condensing it down to z^7n c^n+m + 12 but dont know what word to pump down, as pumping up will just keep increasing m value.