Which two subatomic particles combine to give the atomic mass

The two subatomic particles that combine to give the atomic mass are protons and neutrons.

Atomic mass (sometimes called atomic weight) is the average mass of the atoms of an element. Atomic mass is calculated by taking the sum of the relative abundance of all the isotopes that occur naturally for that element. Isotopes are atoms of the same element that have different numbers of neutrons.  

Atomic mass reflects the size of an atom. Technically, the atomic mass is the sum of the protons, neutrons, and electrons within an atom. However, electrons are so small that their masses are considered to not contribute to the mass of the atom.  

Therefore, the atomic mass of an element is close to the mass number of an element.  The mass number of an atom is the number of protons and neutrons contained within the nucleus of an atom.