The molal freezing point constant for water is 1.86 K.kg mole^-1. The freezing point of 0.1m NaCl solution is

$∆{\mathrm{T}}_{\mathrm{f}}={\mathrm{K}}_{\mathrm{f}}\times \mathrm{m}\times \mathrm{i}$  (i=2 for NaCl $\to$Na⁺ +Cl^-)

M =0.1 molal

K_f =1.86 ⁰C/molal

$∆{\mathrm{T}}_{\mathrm{f}}$=1.86 $\times$0.1$\times$2 =0.372 ⁰C

Therefore, now freezing point =0 - 0.372 ⁰C==-0.372 ⁰C

where i is vant hoff constant/factor.