Answer
$-7.6^{\circ}C$
Work Step by Step
$\Delta T_{b}=m\times K_{b}\implies m=\frac{\Delta T_{b}}{K_{b}}$
$\Delta T_{f}=m\times K_{f}=\frac{\Delta T_{b}}{K_{b}}\times K_{f}$
$=(\frac{2.1^{\circ}C}{0.512}\times1.86)=7.6^{\circ}C$
Freezing point= $0^{\circ}C-7.6^{\circ}C=-7.6^{\circ}C$
