GetTotalProfitByCategory

        public static string GetTotalProfitByCategory(BookShopContext context) //12.
        {
            var result = context.Categories
                .OrderByDescending(c => c.CategoryBooks.Select(cb => cb.Book.Copies * cb.Book.Price).Sum())
                .Select(c => $"{c.Name} ${c.CategoryBooks.Select(cb => cb.Book.Copies * cb.Book.Price).Sum():f2}");

            return String.Join(Environment.NewLine, result);
        }