Question about calculating molar m from freezing point... HELP!!?

A solution is prepared by dissolving 2.59 g of an unknown nonelectrolyte in 23.00 g of cylcohexane (tf = 6.6 oC and kf = 20.0 oC/m). What is the molar m of the unknown compound if the solution has a freezing point of -6.4 oC?