The word EXTENSION can be rearranged into the words ONE, TEN, and SIX.

String together the numbers 1, 10, and 6 and you get 1106.

Add them and you get 17.

The word EXTENSIVELY can be rearranged into the words SIXTY and ELEVEN.

String together the numbers 60 and 11 and you get 6011.

Add them and you get 71.