Question

Enter the appropriate symbol for an isotope of calcium-42 corresponding to the isotope notation AZX.

Answer 1

The proton number number of the element stays constant in its isotopes while the mass number changes. So, for this isotope of Calcium:
42Ca20

Answer 2

Answer is ₂₀⁴²Ca.

In AZX notation
   A - Represents the mass number of atom.
         Mass number = number of protons + number of neutrons.
         It is shown at the above of the left side of the atom.
         Here it is 42. 

  Z - Represents the atomic number.
       Atomic number = number of protons.
       It is shown at the bottom line of the left side of the atom.
       Here it is 20.
 
  X - Represents the symbol of the atom.
        Here X is Ca.