Which two subatomic particles do you add together to determine the atomic mass or mass # of the atom?

Protons and Neutrons

Explanation:

The sum of number of protons and the number of neutrons is equal to the mass of an element. Mathematically, it can be given as :

M = P + N

Also, the atomic mass can be calculated by the sum of the element's isotopes as isotopes have slightly different mass numbers.

Hence, protons and neutrons add together to determine the atomic mass.