What was the aggregate for the celebrity who was known for being a singer and had 7 dances?
CREATE TABLE "list_of_contestants" (
  "celebrity" text,
  "known_for" text,
  "age" real,
  "professional" text,
  "series" real,
  "place" text,
  "dances" real,
  "highest_score" real,
  "lowest_score" real,
  "aggregate" real,
  "average" text
);
SELECT "aggregate" FROM "list_of_contestants" WHERE "dances"=7 AND "known_for"='Singer';