Im 17 and also watching what i eat b/c im overweight,i also take a brisk walk for 20 -25 minutes twice a day. is this enough exercise?

Enough exercise?: There is a balance between calories absorbed and calories burned. The difference between these two result in weight stored or weight loss. If your intake is greater than the energy burned then it is not enough exercise. Slow and steady is better than short cycles of effort followed by frustration and giving up. Good luck. You can do it and it will pay off for the rest of your life.

Well, that depends..: You will be burning roughly 100 calories for each mile you walk. There are 3,500 calories per pound. Let's say you walk 2 miles per day - that is 200 calories. At that rate you would lose about 1 pound every 18 days (if all else was kept the same). See you take phentermine. You will get from your program - what you put into it. Take care.