We know that $\large\frac{P^0-P_s}{P_s}=\frac{w\times M}{m\times w}$
Substituting values,
$\large\frac{100-98.88}{98.88}=\frac{w}{m}\times \frac{78}{W}\times \frac{1000}{1000}$
$\Rightarrow $ Molality $\times \large\frac{78}{1000}$
(or) Molality $(\large\frac{w\times 1000}{m\times W})=\frac{1.12\times 1000}{78\times 98.88}$
$\Rightarrow 0.1452$
We know that
$\Delta T_f=K_f\times$ molality
$\therefore K_f=\large\frac{\Delta T_f}{\text{Molality}}=\frac{0.73}{0.1452}$
$\Rightarrow 5.027Kmolality ^{-1}$