An isotope of yttrium has 39 protons and 59 neutrons. what is the atomic mass of that isotope?

We can define atomic mass the total of number of protons and number of neutrons in an atom or isotope.

So when an isotope of yttrium has 39 protons and 59 neutrons, its atomic mass is equal to;

number of protons + number of neutrons = 39 + 59 = 98