In the nucleus of an atom, there are 18 protons and 20 neutrons. what are the atomic number and mass number of this isotope

The atomic number is the number of protons in the element - just 18. The mass number is the atomic number plus the number of neutrons: 18 + 20 = 38.

An isotope is just an element with a particular number of neutrons, which is different from the element with a different number of neutrons. Element 18 is argon, and this particular isotope would be names Argon-20, or Ar-20