Mega Hard Code

[jackymcjack]

LBLFFT DNU GDA?
HSTC LT TDA QOB PCS BHSF, ERDFP EX FT KEPNP! ><
HSTC WROVHHV! =D

Any ideas?

Clues:
Its using Subtraction.
Letters each have a value.

The first line is:
Having fun yet?

I need the equation and the answer...

help?!