Ordered Customers:

public static string GetOrderedCustomers(CarDealerContext context)
        {
            var customers = context.Customers
                .OrderBy(c => c.BirthDate)
                .ThenBy(c => c.IsYoungDriver == true)
                .Select(c => new
                {
                    c.Name,
                    BirthDate = c.BirthDate.ToString("dd/MM/yyyy", CultureInfo.InvariantCulture),
                    c.IsYoungDriver
                })
                .ToArray();

            var jsonFile = JsonConvert.SerializeObject(customers, Formatting.Indented);

            return jsonFile;
        }

Cars with List Parts:

public static string GetCarsWithTheirListOfParts(CarDealerContext context)
        {
            var cars = context.Cars
                .Select(c => new
                {
                    car = new
                    {
                        Make = c.Make,
                        Model = c.Model,
                        TravelledDistance = c.TravelledDistance
                    },
                    parts = c.PartCars.Select(pc => new
                    {
                        Name = pc.Part.Name,
                        Price = $"{pc.Part.Price:F2}"
                    }).ToArray(),
                }).ToArray();

            var jsonFile = JsonConvert.SerializeObject(cars, Formatting.Indented);

            return jsonFile;
        }

Total Sales by Customer (this is the standard solution, but throws error in VS):

        public static string GetTotalSalesByCustomer(CarDealerContext context)
        {
            var customers = context.Customers
                .Where(c => c.Sales.Count() > 0)
                .Select(c => new
                {
                    fullName = c.Name,
                    boughtCars = c.Sales.Count(),
                    spentMoney = c.Sales.Sum(s => s.Car.PartCars.Sum(pc => pc.Part.Price))
                })
                .OrderByDescending(c => c.spentMoney)
                .ThenByDescending(c => c.boughtCars)
                .ToArray();

            var jsonFile = JsonConvert.SerializeObject(customers, Formatting.Indented);

            return jsonFile;
        }

Sales with Discount:

        public static string GetSalesWithAppliedDiscount(CarDealerContext context)
        {
            var sales = context.Sales
                .Take(10)
                .Select(s => new
                {
                    car = new
                    {
                        Make = s.Car.Make,
                        Model = s.Car.Model,
                        TravelledDistance = s.Car.TravelledDistance
                    },
                    customerName = s.Customer.Name,
                    Discount = $"{s.Discount:F2}",
                    price = $"{s.Car.PartCars.Sum(pc => pc.Part.Price):F2}",
                    priceWithDiscount = $"{s.Car.PartCars.Sum(pc => pc.Part.Price) * (1 - (s.Discount / 100)):F2}",
                })
                .ToArray();

            var jsonFile = JsonConvert.SerializeObject(sales, Formatting.Indented);

            return jsonFile;
        }