$ = 0.687\,^oC = \,\,0.69\,^oC$
$\Delta {T_f}\, = \,\,{({T_f})_{solvent}}\, = \,\,{({T_f})_{solution}}$
$0.69\,\, = \,\,0\,\, - \,\,{({T_f})_{solution}}\,$
$ \Rightarrow \,\,\,{({T_f})_{sol}}\, = \,\, - \,\,0.69\,^oC$