The sun sum of protons and neutrons in the nucleus of an atom.

Answer is the mass of the atom. Protons + neutrons = mass

Let's say you have oxygen with a mass of 16.

Oxygen has 8 protons

To find neutrons subtract 16-8 = 8

So it would also be 8 neutrons.

Example of how to find mass if you know protons + neutrons:

Sodium has 11 protons and 12 neutrons. Add the protons and neutrons

11 + 12 = 23

Mass is 23