How to alert on declining metric?


I want to set an alarm when a metric starts declining. The graph looks something like this:


I would like to know what conditions should I specify?

write a query that graphs the derivative and check if that is negative.

I felt checking if the value is not null after 10 days is a better approach to my use case.
So In “Evaluate query” can I mention “10d” to check every 10 days?
Is it valid?