What is a level of production in which the marginal production decreases with new investment?

The development of a system of mass production in manufacturing meant that the assembly line had a high volume of production rate. It also meant the use of heavy machineries that had changeable parts. Henry Ford was the first person who actually used the theory of mass production to great success in the earlier twentieth century. His success allowed several people to adopt this theory for getting success. The advertisers of that period used the assembly line designs to project several industries.    

The Nash equilibrium is a situation where individuals or players have no incentive to change their strategy taking into account the strategy of their opponents.

So in this case the nash equilibrium is "both advertise" The best equilibrium would be not to advertise any of the 2 but taking into account the attempt of what the opponent can do the nash equilibrium is where both advertise.