Proposed are a method by which a first apparatus performs wireless communication, and an apparatus supporting same. The method may comprise the steps of: obtaining a sidelink (SL) discontinuous reception (DRX) configuration including information related to an SL DRX timer; receiving, from a second apparatus through a physical sidelink control channel (PSCCH), second sidelink control information (SCI) and first SCI for scheduling a physical sidelink shared channel (PSSCH); receiving, from the second apparatus through the PSCCH, the second SCI related to groupcast and a medium access control (MAC) packet data unit (PDU); and determining, on the basis of an index of a sub-channel related to the PSSCH and an index of a slot, a physical sidelink feedback channel (PSFCH) resource.