CREATE TABLE table_21892 ("Outcome" TEXT, "Year" FLOAT, "Championship" TEXT, "Surface" TEXT, "Partner" TEXT, "Opponents" TEXT, "Score" TEXT)

On what surface was the game played with a score of 6 4, 6 4?
SELECT "Surface" FROM table_21892 WHERE "Score" = '6–4, 6–4'
SELECT "Surface" FROM "table_21892" WHERE "Score" = '6–4, 6–4'