How to lose a stone in 2-3 months?

ok, im size 14 and want to become a size 10-12 in 3 months. My meal plan is muesli and an apple for breakfast with a green tea, a banana for a snack, ham salad and a yoghurt drink for lunch, a carrot snack and then tuna salad for dinner. I also drink 6 gles of water a day. For exercise i do 3 sets of 30 step ups , 100 sit ups, 10 minutes skipping and a 30 minute brisk walk a day. I was just wondering if i will acheive my goal by doing this? x