-8400/68 simplifies to to -2100/17, dolphins/year, so 2100 less dolphins every 17 years.
and we can just divide those to get the decimal amount of -123.53, which means, 123.53 less dolphins every year.
of course, there's no such a thing as 0.53 of a dolphin, so, we can say "about" 123 less dolphins per year.