Question

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

Answer 1

So what you will need is a periodic table. The numbers you see on top of the chemical element symbol is the number of protons. The bigger number below the said number is the mass number.
The previous information was the general way/ easier way without math to get what you were looking for but that way wont help as you get into more chemistry.

To answer your question, the atomic number is the number of protons and the mass number is the combination of both the protons and neutrons.

Answer 2

do you think you could solve the problem by yourself or do you need more help?