Magnetic moment of any compound is given by
µ = \(\sqrt { n(n+2) }\)
Where n is the no. of unpaired electrons.
So,
3.9 = \(\sqrt { n(n+2) }\)
Squaring both side
15.21 = n2 + 2n … (1)
This equation (1) satisfying for n = 3. So no. of unpaired electrons in this elements will be three.