For Stellenbosch, which has a population larger than 155,733, what is the average area?
CREATE TABLE "geography" (
  "name" text,
  "seat" text,
  "population_2011" real,
  "area_km_2" real,
  "density_inhabitants_km_2" real
);
SELECT AVG("area_km_2") FROM "geography" WHERE "name"='stellenbosch' AND "population_2011">'155,733';