// problem 6
public static string GetBooksByCategory(BookShopContext context, string input)
        {
            //Return in a single string the titles of books by a given list of categories.
            //Order by title alphabetically.
            string[] categories = input.Split(' ');
            StringBuilder output = new StringBuilder();

            string[] bookTitles = context
                .Books

                 .Where(book => book.BookCategories. ) // the category of the book contains all of the categories listed ??
                                                       // relationship of Books with BooksCategories and Categories
                .Select(book => book.Title)
                .OrderBy(book => book)
                .ToArray();
                foreach(string title in bookTitles)
            {
                output.AppendLine(title);
            }

            return output.ToString().TrimEnd();
        }