The complex [CoF6]3− is an octahedral complex.
Here the oxidation state of Co is +3
Thus the elctronic configuration of Co3+=[Ar]3d6
So, according to crystal field splitting theory, the electronic configuration in d orbital will be t42ge2g
So number of unpaired electrons is = 4
Again spin only magnetic moment is given by, √n(n+2),
Where n =number of unpaired electron
So, the spin only magnetic moment of [CoF6]3+=√4(4+2)=4.89