Explanation

(a)
In ${NO}_{3}^{-}$ ion, total number of electrons = 7+24+1=32 and in it central atom is $sp^{2}$ hybrid.
No. of hybrid orbitals = $\frac{V - 8B}{2} + B = \ \frac{24 - 8 \times 3}{2} + 3$
($V$ $\rightarrow$ total number of electrons in valence shell
$B$ $\rightarrow$ probability of formation of bond)
In ${CO}_{3}^{2 -}$ ion, total number of electrons = 6+24+2=32 and in it central atom is $sp^{2}$ hybrid.
No. of hybrid orbital= $\frac{24 - 8 \times 3}{2} + 3 = 3$
Hence, ${NO}_{3}^{-}$ and ${CO}_{3}^{2 -}$ ions are isoelectronic and isostructural.