Question

How to find the Arimetgic in a sequence

Answer 1

To the "debate on how to do this problem"
Just use the standard form -> nth term= a1 + (n-1)*(D)
in this case
100th= 15 + (100-1) (-6)
100th= -579
where d is the common difference, a1 is the first term and n is the number of terms, then you'll never loose track of negatives!