what is the enrollement for the year joined 1958 in clarksville?
CREATE TABLE "membership" (
  "school" text,
  "location" text,
  "mascot" text,
  "enrollment" real,
  "ihsaa_class" text,
  "num_county" text,
  "year_joined" real
);
SELECT SUM("enrollment") FROM "membership" WHERE "year_joined"=1958 AND "location"='clarksville';