What is the frequency MHz of the ERP W's greater than 250?
CREATE TABLE "broadcast_translators_of_kanz" (
  "call_sign" text,
  "frequency_m_hz" real,
  "city_of_license" text,
  "erp_w" real,
  "class" text,
  "fcc_info" text
);
SELECT AVG("frequency_m_hz") FROM "broadcast_translators_of_kanz" WHERE "erp_w">250;