number of protons: you have to know the atomic number or the name/symbol of the element. If I tell you the atomic number is 11, you know that the number of protons is 11. If I tell you the element is uranium, you look up the atomic number on a periodic table and then you know that the number of protons is 92 because that's the atomic number of uranium.
number of neutrons: you have to know the mass number (which is the sum of the protons and the neutrons) of the given isotope under question. For example, I tell you I have two isotopes of nitrogen, mass numbers of 14 and 15. How many neutrons in each? N-14 ---> 14 minus 7 = 7 and N-15 ---> 15 - 7 = 8. Notice that 7 is the atomic number of nitrogen and I used it in both subtractions.
nmber of electrons: you have to know the charge on the atom. Suppose I have an atom of magnesium and it is neutral. How many electrons? The answer is 12 because there are 12 protons (look up Mg on periodic table) and that means 12 positive charges. Since the atom is neutral, there have to be 12 negative charges, so 12 electrons. Suppose the atom of Mg has a +2 charge, how many electrons? The answer is 10. The 12 pos charges are balanced by 10 neg charges, leaving me with a +2 overall charge on the atom (which is now called an ion, since it has a charge)/
Chlorine-37 with a -1 charge. How many p, n, and e?
p = 17
n = 20
e = 18
Best wishes on the exam.