How do you answer cual su de nacimiento?

Answer 1

It doesn't make sense as written; probably should be:

Cual (es) su (fecha) de nacimiento? = What (is) your/his/her (date) of birth?

To which the answer would be:

Nacio (with an accent on the 'o') el (a number) de (a month) de (a year), e.g.

Nacio el 3 de enero de 1906 = He/she was born on the 3rd of January, 1906

or

(Yo) naci (accent on the 'i') el (your birth-date) de (birth month) de (birth-year) = I was born on the (date) of (month), (year).

Answer 2

"Mi lugar de nacimiento es (nombre de la ciudad/país)."