Which 1997's accompanying 1991 was a when the tournament was the australian open?
CREATE TABLE "mixed_doubles" (
  "tournament" text,
  "1989" text,
  "1990" text,
  "1991" text,
  "1992" text,
  "1993" text,
  "1994" text,
  "1995" text,
  "1996" text,
  "1997" text,
  "career_w_l" text
);
SELECT "1997" FROM "mixed_doubles" WHERE "1991"='a' AND "tournament"='australian open';