In the presence of ligand NH3, pairing does not occur in Ni2+ because pairing of electrons cannot create two vacant d-orbitals. Hence, nickel ion undergoes sp3d2 hybridisation and it has two unpaired electrons.
Ni2+=3d8

No. of unpaired electron =2
The spin only magnetic moment
μ=n(n+2)=8=2.82BM
=28.2×10−1BM
x=28