Of 2010 est. with less than 171,750 and 2000 less than 131,714, what is the 1970 population average?
CREATE TABLE "population_growth_history" (
  "city" text,
  "1970" real,
  "1980" real,
  "1990" real,
  "2000" real,
  "2010_est" real
);
SELECT AVG("1970") FROM "population_growth_history" WHERE "2010_est"='171,750' AND "2000"<'131,714';