What is the Quantity of the engine Model PG-2901?
CREATE TABLE "p_series_1939_1944" (
  "model" text,
  "built" text,
  "quantity" real,
  "seats" real,
  "length" text,
  "engine" text
);
SELECT "quantity" FROM "p_series_1939_1944" WHERE "model"='pg-2901';