What was the surface when the opponent was sergiy stakhovsky mikhail youzhny?
CREATE TABLE "doubles_6_2_4" (
  "outcome" text,
  "date" text,
  "surface" text,
  "partner" text,
  "opponents" text,
  "score" text
);
SELECT "surface" FROM "doubles_6_2_4" WHERE "opponents"='sergiy stakhovsky mikhail youzhny';