What is the average Frequency, when Type is "Norteño", and when Brand is "La Cotorra"?
CREATE TABLE "am" (
  "frequency" real,
  "callsign" text,
  "brand" text,
  "city_of_license" text,
  "type" text
);
SELECT AVG("frequency") FROM "am" WHERE "type"='norteño' AND "brand"='la cotorra';