Here's a possibility using the graph of y = 20sin(πx / 3 + π / 2) + 80
Look at the graph, here : https://www.desmos.com/calculator/derjhfgw9m
At x = 0 (representing year 1999) we have 100 horses
At x = 3 ( representing year 2002 ) we have 60 horses
And again, at x = 6 (representing year 2005 ) we're back up to 100 horses
Sorry, I don't understand what "b" is asking for
c) In 2000 ( x = 1) there were 90 horses
d) In 2015 ( x = 16) there were 70 horses
