Solve this colligative property of sol'n?

A solution is found to contain 0.256 unknown solute and 3.56g naphthalene. The freezing point of the solution is 75.2 degreesC. What is the molar m of the unknown solute if the freezing point of pure naphthalene is 80.6 degreesC? The molal freezing point depression constant for napthalene is Kb=6.9 degreesC/molal.