For this case we have the following sentence:
Las mochilas.
This sentence is written in the plural.
That is, it means that we have more than one object.
We must transform the sentence into singular, that is, a single object. For this, we must eliminate the letter "s" of both words.
We have then:
La mochila
Answer:
the singular form of las mochilas is:
La mochila
