An isotope of potassium contains 19 electrons, 21 neutrons, and 19 protons. What's its atomic mass number?

The total number of nucleons, or the particles in the
nucleus, which includes both the protons as well as the neutrons make up the atomic mass
of the atom. The atomic number is the number of protons in the nucleus, which for a
neutral atom is equal to the number of electrons revolving around the
nucleus.

Here you have provided the details for potassium
which has the number of electrons as 19, number of protons as 19 and the number of
neutrons as 21.

The atomic number is therefore 19 and the
atomic mass is 19 + 21 = 40.

The required
atomic mass of potassium is 40.