How many positive AND negative factors does 3600 have?
Why is it 90, not 89? 0 is neither positive nor negative.
0 is not a factor of 3600, so it’s not part of the 90.
For a number a to be a factor of b, there must exist an integer q such that b = aq.
In your case, that would mean 3600 = 0 \cdot q. Since 0 \cdot q is always 0, no such q exists. Therefore, 0 is not a factor of 3600.
Maybe you’re confusing yourself with the converse?
The converse being the following:
3600 is a factor of 0
This is a true statement because 0 = 3600 \cdot q holds true when q = 0, which is a valid choice for q.