The molal cryoscopic constant for benzene will be equal to
You're dealing with a solution that consists of naphthalene, your solute, and benzene, your solvent. In order to determine the cryoscopic constant of benzene, you'll need to calculate the molality of the solution.
Molality is defined as moles of solute per kilogram of solvent. Use naphthalene's molar mass to determine how many moles you have
The molality of the solution will thus be
The freezing point depression equation looks like this
Since you're dealing with a non-electrolyte, the van't Hoff factor will be equal to 1.
Since you know the freezing point of benzene and of the solution, you can write
Rounded to two sig figs, the number of sig figs you gave for the freezing points, the answer will be
SIDE NOTE The value of the cryoscopic constant for benzene is actually