The difference of a number times 2 and 8 equals 7 (translate)

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