You're going to need a good diet AND good exercise to get that much off that quickly. It's possible, and I think that you can do it, but you need to be doing things right.
What does your current diet/fitness regimen look like? Also, have you figured out what your daily caloric maintenance requirements look like?(you can find out a rough number from a calculator like this one: http://www.exrx.net/Calculators/CalRequire.html