What year did niels bohr discover the atomic theory​

1913

Explanation:

A hypothesis of the hydrogen atom was developed by Niels Bohr in 1913, based on the quantum theory, which states that certain physical quantities can only have discrete values. Electronic particles travel about a nucleus, but only in specified orbits. If electrons jump to a lower-energy orbit, the energy difference between the two orbits is emitted as radiation.

Hope this helps explain what the answer to your question is and why it is that answer.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.