I will assume the statement is supposed to be "the difference between 8 times a number and 5",
The important part of this problem is to note the formal mathematical definition of "difference".
In everyday English, the difference between 5 and 10 is 5, and the difference between 10 and 5 is 5. That's because the usage does not distinguish between the larger and smaller number; the "difference" is taken to be a positive number.
That can't be the case in formal mathematics. In mathematical language, the "difference between A and B" is A-B. So in mathematical language, the difference between 5 and 10 is 5-10 = -5, while the difference between 10 and 5 is 10-5=5.
So your (corrected) statement is the difference between 8n and 5, which is