You find the atomic mass of an element by adding the protons and the neutrons, therefore in order to find the atomic mass for this element you add 19 and 21
19 + 21 = 40
The atomic mass is 40.
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.