This is actually really is. Notice how the protons + the neutrons equal the top number next to the element and the bottom one is the amount of protons.
So for Rn, it already gives you the protons and the atomic number (the top number), you just subtract them ---> 209-86= 123 neutrons.
I don't have a periodic table to look at, but for the next one just add the protons and the neutrons to get the atomic number (the atomic number will be under the element symbol) and look for it on the periodic table to find the element and so on.
