*** NOT A TEACHER ***
I apologise for not having waited for a teacher/native speaker, but you might find these links useful:
1.) Used to: questions and negative forms
For questions and negative forms, two forms of the verb are used - either the normal infinitive pattern after did (more common), or the past form used (less common):
When you were a kid, did you use to think the sun revolved around the earth?
When you were a kid, did you used to think the sun revolved around the earth?
I didn't use to take such a large dress size, but now I do.
I didn't used to take such a large dress size, but now I do.
(By Roger Woodham from BBC Learning English)
2.) 1.) didn't used to...? (WordReference.com)
It is a bit of a consolation that it seems to puzzle even some native speakers.
3.) Didn't used to vs. didn't use to?? (An old thread from this very site
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