Final answer:
In Spanish, the form of 'grande' used with 'mochilas' depends on the context. It would be 'grande' for one backpack and 'grandes' for multiple backpacks.
Step-by-step explanation:
In Spanish, the word for 'big' is 'grande'. However, the form of 'grande' that you would use with 'mochilas' (which means 'backpacks') depends on the specific context of the sentence. If you are referring to one backpack, you would use the singular form 'grande'. For example, 'una mochila grande' means 'a big backpack'. If you are referring to multiple backpacks, you would use the plural form 'grandes'. For example, 'dos mochilas grandes' means 'two big backpacks'.