CREATE TABLE table_28019988_5 (season__number INT, us_viewers__million_ VARCHAR, written_by VARCHAR)

What episode number of the season was written by Tom Garrigus and had 2.31 million u.s. viewers?
SELECT MAX(season__number) FROM table_28019988_5 WHERE us_viewers__million_ = "2.31" AND written_by = "Tom Garrigus"
SELECT MAX("season__number") FROM "table_28019988_5" WHERE "2.31" = "us_viewers__million_" AND "Tom Garrigus" = "written_by"