What is the lowest share with an air date of March 21, 2008 and had viewers larger than 5.15?
CREATE TABLE "u_s_standard_ratings" (
  "air_date" text,
  "timeslot" text,
  "rating" real,
  "share" real,
  "18_49" text,
  "viewers" real,
  "weekly_rank" text
);
SELECT MIN("share") FROM "u_s_standard_ratings" WHERE "air_date"='march 21, 2008' AND "viewers">5.15;