Which Weight has a Power of kw (hp) at 1,800rpm, and a Name of 9 nc?
CREATE TABLE "salmson_post_world_war_one_engines" (
  "name" text,
  "cyl" text,
  "bore" text,
  "capacity" text,
  "power" text,
  "weight" text
);
SELECT "weight" FROM "salmson_post_world_war_one_engines" WHERE "power"='kw (hp) at 1,800rpm' AND "name"='9 nc';