Most Helpful Guy
Everyone has a six pack. There's just usually a layer of fat covering it up. Although strengthening your stomach muscles is good, you should really focus on losing fat. Losing weight would also certainly help your back. There are no truly good exercises that you can do that won't put stress on your back. You should really try some exercises to strengthen your back. A strong back is a happy back.
Is there a specific reason your back bothers you that you know of? A friend of mine hurt his back badly once and had to have surgery. In a case like that, you want to consult your doctor before you take advice from people on this website.
Speaking of websites.. bodybuilding.com is an Amazing informational website for health and fitness. There you can find dieting and exercise info from qualified professionals.
Now for what you don't want to hear. If your back just bothers you and you have had no Serious injuries or birth/growth related problems, I'd say you're just making excuses for yourself. Stop it.
1: Focus on setting up and sticking to a healthy diet. Check out that website.
2: Buy a gym membership. Most gyms offer free personal training to get you started.
It may seem impossible, but it really isn't. If getting to where you wanted to be was easy, everyone would be doing it.
Best of luck.
..and don't forget.. Bodybuilding.com Don't be fooled by the name. There's thousands of useful tips on health, fitness, and life on there.