PS5.8 a asks you to determine the molality. This part of the problem is pretty straight forward. Part b is the challenging part of this problem. I will not tell you exactly how to do part b, but here is a hint.

Density of a solution is defined as the grams of the solution/volume of the solution. What concentration term that we discussed in class contains grams of solution? What concentration term that we discussed in class contains volume of solution? OK now can you arrange those two concentration terms to obtain the density of the solution?

Hope this helps.