A solution exhibits colligative properties since solute interferes with the interaction of the atoms which would change certain properties of the solution. In this case, it is the freezing point that is affected. We calculate as follows:
ΔT = kf (m) i
where kf is the freezing point depression constant which is 1.86 for water
m is molality
i is the van't hoff factor which is 2 for NaCl
ΔT = 1.86(0.321) 2
ΔT = 1.19412 °C
1.19412 °C = 0 - T
T = -1.19412 °C