CREATE TABLE table_43084 ("Usage" TEXT, "Sn/Sb  ( % ) " TEXT, "Liquid at  ( \\u00b0C ) " TEXT, "Casting at  ( \\u00b0C ) " TEXT, "Remelting at  ( \\u00b0C ) " TEXT, "Hardness" TEXT)

What is the casting temperature for the alloy with hardness 21?
SELECT "Casting at (\u00b0C)" FROM table_43084 WHERE "Hardness" = '21'
SELECT "Casting at (\u00b0C)" FROM "table_43084" WHERE "Hardness" = '21'