Select 1 Dragon-Type Synchro Monster in your Graveyard. Remove from play other monsters in your Graveyard whose total Levels equal the Level of the selected monster, and Special Summon it from the Graveyard.
.....BRD is a Dragon-Type. Perhaps don't try to call yourself Yusei if you don't even know anything about the Signer Dragons.
As for the question, do you have other monsters in your Graveyard you can remove, whose total level is exactly 7? It cannot be more, so you can't remove a level 4 and another level 4, since the total would be 8 while BRD is level 7.
Does anyone know how to send a Synchro monster like Stardust Dragon straight to the graveyard, from the extra deck instead of the field.