Here's what you could write.
The idea here is that you need to group the cations, which are positively charged ions, with the anions, which are negatively charged ions, to form a neutral ionic compound.
So, your two cations are
#"Ca"^(2+) ->#calcium cations;
#"Cr"^(3+) ->#chromium(III) ations;
and your two anions are
#"O"^(2-) ->#oxide anions;
#"NO"_3^(-) ->#nitrate anions.
Now all you have to do is pair these ions up to form neutral compounds. The key here is to make sure that the charge on the cations is balanced with the charge on the anions.
So, the calcium cations can bond with the oxide anions to form calcium oxide,
#"Ca"^(2+) + "O"^(2-) -> "CaO"#
The calcium cations can bond with the nitrate anions to form calcium nitrate,
This basically tells you that the charge of the anion (in absolute value) becomes a subscript for the cation, and vice versa.
In essence, this means that you need two nitrate anions to balance the charge of one calcium cation.
In this case, you have
#"Ca"^(color(red)(2+)) + color(red)(2)"NO"_3^color(blue)(1-) -> "Ca"_color(blue)(1)("NO"_3)_color(red)(2) = "Ca"("NO"_3)_2#
Use the same technique to find the other two ionic compounds that can be formed with the chromium(III) cations, chromium(III) oxide and chromium(III) nitrate.