Answer
See explanation
Work Step by Step
Resonance structures and best Lewis structures:
a. $NCO^-$:
Resonance structures:
N≡C-O$^-$
$^-$N=C=O
N=C≡O$^-$
Using formal charge arguments, the best Lewis structure for NCO$^-$ is the second resonance structure $^-$N=C=O.
b. $CNO^-$
Resonance structures:
$^-$C≡N-O
C=N$^-$-O
C=N=O$^-$
Using formal charge arguments, the best Lewis structure for CNO$^-$ is the second resonance structure C=N$^-$-O.
