Since you created an integrity constraint on the author_books table with a foreign key, you need to delete author's publications before you delete him from the authors table.
The following code is the DML to Delete Publications.
DELETE FROM author_books WHERE id = 300;
The syntax for the DELETE statement is as follows:
DELETE FROM <table_name> WHERE <column_name_N> = <column_value_N>...;
<table_name> is the name of the table to DELETE FROM <column_name> is one or more columns for which you specify some criteria.
Here is the DML for Deleting Darwen from the Authors Table.
DELETE FROM authors WHERE id = 300;