When to use -masu forms of -i adjectives as opposed to -kunaidesu?


I am doing the exercises for a course in Japanese, which I cannot attend,
because it overlaps with my mandatory course. The exercise requires to form
the past negative masu form of i-adjectives. Instead of

> やすくなかったです

It requires us to answer

> やすくありませんでした

[![enter image description
here](https://i.stack.imgur.com/k53dX.jpg)](https://i.stack.imgur.com/k53dX.jpg)

When to use this form? I suspect it is just an even more polite version, but I
searched a lot and cannot find any example of usage for this, so I am somewhat
insecure!

Edit: When I learned Japanese in the past (from books) the verb-masu form was
always introduced together with the adjective-desu/adjective-kunai desu form.
So, as regards to this new adjective-ku-arimasen, when should I prefer it? And
should I pair it with verb-masu or with something more polite?

