Calculate CFSE for [FeF6]3-?

In [FeF₆]^3- 

x + 6(-1) = -3

x - 6 = -3

x = -3 + 6 = 3

Fe³⁺: [Ar]3d⁵

There is splitting of d-orbital in octahedral complex. (F^- is a weak ligand)

Configuration: t_2g³e_g² 

So, CFSE = (-4 x 3)Dq + (+6 x 2)Dq = 0