I realize that solution of 3.40 is not correct. Actualy you solved for going from y to cost function as it is done in the book but in the question they ask us going from cost to y. So in order solve it you first must see that cost function is the same thing (almost) with expenditure function and there is a question in chapter 2.5 and its solution is in this web site so you can look at it for the solution for cob douglas case.