What is a descending arithmetic sequence?
It's a sequence of numbers that go down in a regular, linear fashion.
An example is 10,9,8,7,... that goes down 1 every step or step=
But 1000, 950, 900, 850... would also be one, because this goes down 50 every step, or step=
These steps are called the 'common difference'.
An arithmetic sequence has a constant difference between two steps. This can be positive, or (in your case) negative.