What is average frequency MHZ when is in portales, new mexico?
CREATE TABLE "new_mexico" (
  "call_sign" text,
  "frequency_m_hz" real,
  "city_of_license" text,
  "erp_w" real,
  "fcc_info" text
);
SELECT AVG("frequency_m_hz") FROM "new_mexico" WHERE "city_of_license"='portales, new mexico';