${\left[\mathrm{Co}(\mathrm{CN}{)}_6\right]}^{4-}$
$\mathrm{x}+6\times (-1)=-4$
$\boxed{\mathrm{x}=+2}$
${\mathrm{Co}}^{2+}:\left[\mathrm{Ar}\right]3{ \mathrm{d}}^7$ and ${\mathrm{CN}}^{-}$ is a strong field ligand which can pair electron of central atom.

It has one unpaired electron (n) in $4 \mathrm{d}-$subshell. So spin only magnetic moment $\left(\mathrm{\mu }\right)=$
$\sqrt{\mathrm{n}(\mathrm{n}+2)}\mathrm{B}·\mathrm{M}$
where $\mathrm{n}=$ number of unpaired electrons.
$\mu =\sqrt{3}$ $\mathrm{B}.\mathrm{M}$
$\mathrm{\mu }=1.73 \mathrm{BM} \simeq 2 \mathrm{BM}$