Draw the Lewis dot structures (difficult to do on this site without using the video option). I will do a few so you get the idea.
SeO2. Lewis dot would be something like this. O=Se-O with 1 lone pair on the Se, 2 lone pairs on the double bonded O and 3 lone pairs on the single bonded O. This will show resonance because you can switch the double bond to the other O atom.
N3-. There are a total of 16 valence electrons so the Lewis dot would look like this. N = N = N with no lone pairs on the central N atom, and 2 lone pairs on each of the other N atoms. This would not show resonance.