Please provide full information whenever and wherever you’re posting a query. Its quite frustrating to try to understand where you’re not provided with enough details.
The missing detail must be that the required trend line is for the year 20X9, so that if 20X0 = 0, then 20X1 = 1, 20X2 = 2 and similarly 20X9 = 9 (i.e 9th period starting from 20X0).
I’m sure you’ll now be able to answer the question. If you have any query, feel free to ask in this forum.