The relative atomic mass of an element is defined as the weight in grams of the number of atoms of the element contained in 12.00 g of carbon-12. To calculate the relative atomic mass of chlorine, the average mass of one atom of chlorine is found by considering 100 atoms of chlorine.
You can find the relative atomic mass by adding the number of protons to the number of neutrons for specific consideration. For example, a carbon-12 atom has 6 protons and 6 neutrons, and so has a relative atomic mass of 12.
I hope this helps