How would you say "i used to sing" using the verb cantar? just write the the subject.

Explanation:

The setence "I used to sing", it's refering to an activity in the past, the subject sang before, but not know, this can be expressed in Spanish as:

"Solía cantar"

If you wanna use the subject would be :"Yo solía cantar".

These statements indicates the same intention as the given sentence.

Therefore, the best answer here is "Solía cantar".