How to answer additive concentration questions

If one assumes the volumes are additive, what is the concentration, in mol/L of (NO3)- in a solution obtained by mixing 281mL of 0.213M KNO3, 333mL of 0.443M Mg(NO3)2, and 747mL of H2O?

total moles nitrate / total volume
[{.282}{.213} + {.333}{.443}{2}]/[.281 + .333 + .747]