How to calculate the number of atoms of each element in a urea?
You were correct in converting the grams of urea to mol of urea, but the next step would be to convert the mol of urea to molecules of urea using avogadro's number (which converts mol to molecules or atoms).  Then you want to multiply the molecules of urea by the number of atoms of the element in question that are present in Urea.  So for example there are 2 atoms of N per molecule of Urea, therefore multiplying the molecules of urea you calculated earlier by 2 will yield the total number on nitrogen atoms present.  As was mentioned in the comments, the value for N atoms present given by your book was incorrect and should have been 3.37 × 10^26 atoms of Nitrogen.  See if you can calculate the correct number of each of the remaining elements, as the answers given were correct.