don’t waste your time doing crunches. Keep in mind when you are working out that you need more than spot training to reach your goals. Working large muscle groups (quadriceps for example) will recruit more muscle fibers and thus burn more energy aka calories. You need to burn calories to get slim and trim.
Exercises to get a flat belly include squats, lunges, push ups and cardio. Of course you need to strengthen your core muscles but if you want to see those muscles, work the big muscle groups and move more.