10g of an unknown compound are added to water to form a 7.89 molar solution. if 2 liters of solution are present, what is the molar mass of the unknown compound?

10 / (7.89) (2) 

molarity equation is M = Moles of solute / Liters of solution

7.89 = Moles of solute / 2

(7.89)(2). The question asks for the molar mass which is defined as g/mol. Dividing would give you the unknown compound. 

MM = 10g / (7.89)(2)