'Nuclear charge' is simply the elements atomic number and is always positive. So the information regarding argons number of neutrons is irrelevant. No matter which isotope of an element you are discussing, the 'nuclear charge' will never change. Therefore the answer is 18+. Now, if you are actually looking for the atomic charge you can find this by taking protons (atomic number) and subtracting number of electrons(for an element). To find charge of an ion you can easily refer to the periodic table where you may already know, group 1 is a 1+ charge, group 2 is a 2+ charge, etc. but in this case we are looking at Argon which is a gas so will not be ionic.