Let, difference of a number = D
The question is unclear, one does not know what "and" means. Assuming the and means add.
Assuming translating this means putting this into something you understand, such as an equation:
D*2 + 8 = 7 <- this is what I think you mean
This can then be solved for the difference of a number
2D + 8 = 7
2D = -1
D = -1/2
