Table: tvb_series_2008_2
Columns: airing_date
english_title_chinese_title
number_of_episodes
hd_format
highest_average_point_ratings
genre
official_website

Question: What was the highest average point rating for a modern suspense show with 21 episodes?
SQL: SELECT SUM("highest_average_point_ratings") FROM tvb_series_2008_2 WHERE "genre" = 'modern suspense' AND "number_of_episodes" = 21