What is the zone of the game played against Estonia?
CREATE TABLE "fed_cup_doubles_performances_10" (
  "edition" real,
  "zone" text,
  "round" text,
  "date" text,
  "against" text,
  "surface" text,
  "partner" text,
  "opponents" text,
  "result" text
);
SELECT "zone" FROM "fed_cup_doubles_performances_10" WHERE "against"='estonia';