The word mochila is feminine in Spanish. This means that it takes feminine articles and adjectives. For example, you would say la mochila (the backpack) and una mochila roja (a red backpack).
In Spanish, nouns are either masculine or feminine, and this gender affects the form of the articles and adjectives used with them. The general rule is that nouns ending in -a are feminine, while those ending in -o are masculine. However, there are exceptions, and mochila is one of the nouns that follows the general rule.