What is the average Frequency MHz that is on farwell, texas?
CREATE TABLE "external_links" (
  "call_sign" text,
  "frequency_m_hz" real,
  "city_of_license" text,
  "erp_w" text,
  "class" text,
  "fcc_info" text
);
SELECT AVG("frequency_m_hz") FROM "external_links" WHERE "city_of_license"='farwell, texas';