CREATE TABLE table_34218 ("Callsign" TEXT, "Area served" TEXT, "Frequency" TEXT, "Band" TEXT, "On-air ID" TEXT, "Purpose" TEXT)

what is the callsign whtat uses 7bu?
SELECT "Callsign" FROM table_34218 WHERE "On-air ID" = '7bu'
SELECT "Callsign" FROM "table_34218" WHERE "On-air ID" = '7bu'