The mass number of an atom is defined as the sum of the number of protons and the number of neutrons in that atom.
So, if m is the mass number, p is the number of protons and n is the number of neutrons, we have:
[tex]m=p+n[/tex]We have that:
[tex]\begin{gathered} p=30 \\ n=34 \\ m=30+34=64 \end{gathered}[/tex]So, the mass number is 64.