To display the ACN from the requesting Agency, rather than the primary ACN for the Case, you will need to do the following:
Ensure that you have the Agc_Reps table in the main report (linked to the Requests table on Requests.req_rep_id = Agc_Reps.rep_id).
Create a sub-report that only has the Case_Agc table.
Link the sub-report on the following fields:
Requests.case_id = Case_Agc.case_id
Agc_Reps.agc_id = Case_Agc.agc_id
Format the sub-report as desired to return the ACN.
Note: If you have more than one instance of the Agc_Reps table in your main report, ensure that you are using the correct instance of the table when you link the sub-report.