Losing weight that quickly is not safe and phentermine is not a long term solution.
Both my daughter and her husband were on the phentermine for a while; they both lost fabulously, but within a month of going off it, they both gained back everything they'd lost, plus some. Both are now obese, where they weren't before; just overweight.
Most weight loss products don't work and none of them are meant for long term use, so even if you lose weight on them, chances are you will gain it back.
First rule out any health issues that might cause weight gain/prevent loss, such as hypothyroidism, insulin resistance, PCOS, etc, then work on cutting calories and getting plenty of exercise. You have almost 6 months, if you start a meal/exercise plan and stick with it, you should be able to lose the weight by Aug, provided you don't have any untreated medical issues, holding you back. If you haven't been tested for any of those things, you should be.
Lots of fruits, veggies, lean meats, etc. Stay away from white sugar, bread, flour, etc and opt for whole grains, low/no fat dairy.