Hello Devika. See, going by the books, you would need 50% marks in class 12 along with NATA Score. Now, since you have 45 marks in maths, it will have a negative impact on overall score. However, you still have an option of applying in the private universities which offer B Arch. They will check only your score in class 12 and not specifically your maths score. You should search for the new private universities which offer B Arch, and contact their admission office. I am sure,  you can get admission.

All the best.