Product (maker, model, type)
PC (model, speed, ram , hd, screen, price)
Laptop ( model, speed, ram, hd, screen, price)
Printer (model, color, type ,price)

with model being the key for everything.
Find those manufacturers that sell Laptops but not PC's

SELECT maker
FROM Product p1
WHERE EXISTS( SELECT *
              FROM Product
              WHERE type = p1.type AND type = 'laptop')
      AND
      NOT EXISTS( SELECT *
                  FROM Product
                  WHERE type = p1.type AND type = 'pc');