[interesting]

Question is write for the pair of ions its ionic compound formulae:

Ca2+ and PO43-
(3- is nominator and 4 is denominator)

Any guidance on this is appreciated.

[pear]

For future reference, please use the sup/sub options above the text-box to denote charge and atom-count.

                                                      Ca²⁺+ PO₄^3-    ??

The key is to find the LCM of the ions' charge so, when they are combined, give you the net charge (unless otherwise stated, the net charge is 0).
     In this case, 2 & 3's LCM is 6.  So we need to have +6 and -6.
Then assign quantity to each atom as to achieve that charge.
     3(Ca²⁺) & 2(PO₄^3-)
For monoatomic ions, assign the # as subtext; for polyatomic ions, surround the ion with parenthesis and apply the # outside of the parenthesis.  
     Ca₃(PO₄)₂