To use the SFP+ transceiver, SFP-10G-SR, in your X2 slot, you need to purchase a Cisco OneX converter,
CVR-X2-SFP10G=. This solution allows you to start the migration to the smaller form factor 10G transceiver being used on most of the newer hardware.
Otherwise, you could use the standard X2 transceiver, X2-10GB-SR.
Good luck!