Not sure what you mean by overall equation - you are asked about the reaction of the nitrogen oxidation, and you have to express it using two given equations. If done correctly, there will be no carbon nor nitrogen left, you will be left just with the original equation.
-first:
N2 + CO = C + N2O
+second:
C + 1/2O2 = CO
-first+second:
N2 + 1/2O2 = N2O