Here's why:
* "Ser" conjugation: The verb "ser" (to be) conjugates differently depending on the subject.
* "Andrés y yo" is a plural subject (we) in Spanish.
* "Somos" is the correct conjugation of "ser" for the plural subject "we" in Spanish.
So, the complete sentence would be: "Andrés y yo somos de la República Dominicana."