Dear aspirant

I think mathematics will be your additional subject as even without failing your overall class 12th status is passed.

As for NEET eligibility you must have PCB and English as your subjects in class 12th and you must have passed all the subjects both in practical and theory.

For NEET eligibility PCB aggregate marks must be 50% or more for Gen category students and 40% for OBC/SC/ST category.

As you said you have aggregate marks of 52% in PCB subject which makes you eligible for NEET even of you fail your additional subject .

You can check eligibility criteria for neet entrance exam from link below

https://medicine.careers360.com/articles/neet-eligibility-criteria-2022/amp

Hope this information helps you

All the best