Explanation

(b)
Given , $T_{f} = - 0.186,\ \ \ \ \ \ \ \ \mathrm{\Delta}Tb\ = ?$
$k_{f} = 1.86$
$k_{b} = 0.512$
We know ,
$\mathrm{\Delta}T_{f} = k_{f} \times m$
0.186=1.86$\times m\ \ \ \ \ \ \ \$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ m = \frac{0.186}{1.86} = 0.1$
so, $\mathrm{\Delta}T_{b} = k_{b} \times m$
$\mathrm{\Delta}T_{b} = 0.521 \times 0.1$
=0.0521${^\circ}$