Where the length is 91', what is the year built?
CREATE TABLE "inland_steam_and_river_boats_of_coos_bay" (
  "name" text,
  "type" text,
  "year_built" real,
  "where_built" text,
  "length" text
);
SELECT "year_built" FROM "inland_steam_and_river_boats_of_coos_bay" WHERE "length"='91''';