Many saw the main issue of expansion of territory allowing slavery as one of economics. the slaves would: provide free labor and therefore an economic advantage to slaveholding states. let white men build businesses rather than work on the farm. soon overrun the states without any knowledge of how to run a government. not compete fairly with white youth seeking employment.

The slaves would:

provide free labor and therefore an economic advantage to slaveholding states.

This is correct. It was a concern that those who were not slave owners would not be able to compete with those who were.

let white men build businesses rather than work on the farm.

This was also a concern. Theoretically, by enslaving others and making them work, white men would have a lot of time to build businesses.

not compete fairly with white youth seeking employment.

This was also a problem. Poor whites rarely had long contracts.

The only one who doesn't fit is:

soon overrun the states without any knowledge of how to run a government.

This wasn't a concern because it has nothing to do with how to run a government or being able to.

regarding the law, the 15th amendment was specifically designed thinking in the rights of black free men.

even though ratified on february 3, 1870, the promise of the 15th amendment would not be fully applied for a long time.poll taxes and literacy tests would allow southern states disenfranchise african americans. only in 1965 the voting rights for african americans would be actually able to vote in the south.

in 1965, president johnson signed the voting rights act, which aimed to overcome legal barriers that prevented african americans from voting as it was guaranteed by the 15th amendment to the u.s. constitution.