$\mathrm{n}=$ number of unpaired electron at central atom.

${\left[\mathrm{Cr}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}$ : ${\mathrm{Cr}}^{3+}$ is the central metal ion.

${\mathrm{Cr}}^{3+}$ : $3{\mathrm{d}}^3$ configuration

 It has three unpaired electrons.

$\mathrm{\mu }=\sqrt{3\left(3+2\right)} \mathrm{BM}=3.87 \mathrm{BM}$

${\left[{\mathrm{CuF}}_6\right]}^{3-}$ : ${\mathrm{Cu}}^{3+}$ is the central metal ion.

${\mathrm{Cu}}^{3+}$ : $3{\mathrm{d}}^8$ configuration

Fluorine is the weak field ligand. Its electronic configuration is ${\mathrm{t}}_{2\mathrm{g}}^6 {\mathrm{e}}_{\mathrm{g}}^2$. Hence, it has two unpaired electrons.

$\mathrm{\mu }=\sqrt{2\left(2+2\right)} \mathrm{BM}=2.84\mathrm{BM}$