What is the highest rank with the notes of sa/b, and a time of 6:39.07?
CREATE TABLE "repechage_2" (
  "rank" real,
  "rowers" text,
  "country" text,
  "time" text,
  "notes" text
);
SELECT MAX("rank") FROM "repechage_2" WHERE "notes"='sa/b' AND "time"='6:39.07';