Why do you have to put el at the end of a word without an a at the end of the word?

In Spanish, words can be of 2 genders: masculine or feminine. The "la" article is used for feminine nouns (la escuela, la taza, etc), and the "el" article is used for masculine nouns (el hotel, el supermercado, etc).
If the noun ends in -a, it is probably feminine (there are exceptions, such as el mapa and el agua, but this is generally the rule).

Bolsa can be referring  to any type of bag,

Estudiar  es la respuesta