How is the melting point of a substance related to its freezing point?
They're the same, if it's a pure compound.
If you heat up a solid compound, its temperature will rise. At a certain temperature, it will start to melt, and the temperature will stay constant untill it's all liquid; only then it will rise further.
Cooling that same liquid will lower the temperature, untill it reaches exactly the same temperature. And then it will start to solidify, while -again- the temperature stays constant untill all of it is solid.