Answer:
a. How did you use to spend the winter evenings?
b. I used to be in love with her.
c. They used to come out in those days.
Step-by-step explanation:
"Used to" is being used in a sentence when you're stating a habit or condition that happened in the past; so this means that it is not happening anymore at the moment.
When using "used to" in questions, make sure to use the verb's base form. Thus, you have to say "How did you use to spend..." rather than "How did you used to spend..."
Another example: "Did you use to dance in school?"
On the other hand, when using "used to" in a sentence (other than the interrogative), you have to use "used to." Just like the answers for b & c.