It’s asking what subatomic particles each could have where one would be negative and they’d have the same mass. Since one is neutral it will have an equal amount of electrons and protons, while the negative one will have more electrons since they are negatively charged. It would be possible for one atom to have 9 protons, 9 electrons, and 1 neutron while the other has 9 protons and 10 electrons. Or you could take out the neutron because electrons don’t really have mass