If the position in table is 10th, and the manner of departure was resigned, what was the date of vacancy?
CREATE TABLE "managerial_changes" (
  "team" text,
  "outgoing_head_coach" text,
  "manner_of_departure" text,
  "date_of_vacancy" text,
  "position_in_table" text,
  "incoming_head_coach" text,
  "date_of_appointment" text
);
SELECT "date_of_vacancy" FROM "managerial_changes" WHERE "manner_of_departure"='Resigned' AND "position_in_table"='10th';