Why would the ftc and the justice department be concerned about the merger of two strong companies in a market with few competitors? what advantages could the newly-formed company have?

Because there is no competition, there is no great
incentive to control cost or to use resources efficiently.
There is no need to spend much money on research
and development, to improve processes, to develop new products, or to be responsive to customer needs.

And an advantage could the newly -formed company have is that the monopolist can charge higher prices and provide poorer quality and service.
poorer quality and service.

Answer(say);