2 more problems:

I insert the above in brackets into the combination formula_{n}C_{k }which becomes: $\displaystyle \frac {n!}{(n-2)!(n-(n-2))!}=6$

solving for n I get 1.12 or 3.33, book gives the answer of 4.

I am incorrect for the second question(45) as well using the same method, my answer is 0/1/-7/-8/-9(approximated), book answer is 7.