Given :
$W_1=20.2g$
$W_2=0.2g$
$\Delta T_f=0.45^{\large\circ}C$
$K_f=5.12Kkg mol^{-1}$
Molar mass of acetic acid $M_2=\large\frac{K_f \times W_2\times 1000}{\Delta T_f\times W_1}$
$\Rightarrow \large\frac{5.12\times 0.2 \times 1000}{0.45\times 20.0}$
$\Rightarrow 113.77gmol^{-1}$