Molarity and molality are both used to express the concentration of a solution. The amount of solute is both expressed in moles.
Their major difference is for Molarity, it is expressed as the moles per Liter/s of solution while for Molality its moles per Kilogram/s of solvent.
Molarity (M) = moles of solute / Liter(s) of solution
Molality (m) = moles of solute / Kilogram(s) of solvent
Take not of the use of symbols (capital letter M for molarity and small letter m for molality)
To avoid confusion, always remember that:
Volume of solution = volume of solute + Volume of solvent
Kilogram of Solution = Kilogram of solute + Kilogram of solvent
Tip: you can use the density of the solution in working with this kind of problem.