Posted 02 December 2013 - 04:24 PM
Looks like you're using the second edition. One of the many small things we added to the third edition is a hand-written annotation on that page that explains the result. Here's what it says:
The length is 21 because “Hello!” is six characters, plus the \n counts as another character, which gives 7 x 3 = 21.
Author, Head First C#
Building Better Software -- http://www.stellman-greene.com