Explanation:
(I can't help you for your screenshot, sorry! :D)
An oxygen atom with a negative 2 charge will have 2 more electrons than protons. Since the number of protons in oxygen is 8, this gives 10 electrons.
The molar mass of oxygen is known to be 16g/mol, so the other 8 comes from the neutrons.
Hence there are 8 protons, 8 neutrons, 10 electrons, and 16g/mol.
