How to get Purch.Ord. number from trans. /Nmir5 once you have Inv.Doc.nr.

Does anyone knows how to get in a program PurchaseOrder number from transaction /Nmir5 once you have Inv.Doc.nr.(Invoice nr)

I was looking for tables having data element RE_BELNR but did not found a suitable table(maybe more tables are included)

Thank you in advance . I will be glad to award solution