What is the average frequency in MHz for stations with an ERP W of 170?
CREATE TABLE "kansas" (
  "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 "kansas" WHERE "erp_w"=170;