You can check that your formula is right by substituting some obvious values for x.
So after 1 year the population should be 7900 as it has decreased by 100 people in that year. Your formula gives -8080.
The right answer to a is 8000-100x,
and b is .
I don't like just giving people answers but I don't really know a process to get these; I just know them. sorry.