How to find the Arimetgic in a sequence

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!